Years ago, when I was a law student in a moot court competition, our imaginary client was in need of an argument. Factually and legally, our side had been put in one of those tight corners for which the Jessup Moot is notorious. We asked our professor/advisor for assistance. Never wishing to "spoon feed" or be didactic, he suggested we check whether an "estoppel" point was available. "Check Temple of Preah Vihear – or what McNair had to say on the matter," he added cryptically.

And thus, with some help from a hard copy of the Max Planck Encyclopedia of Public International Law (the internet not having been invented, much less Google and Wikipedia) we stumbled upon the said Temple case, suggesting that "estoppel" indeed was a viable doctrine in international law,[1] as well as the famous dictum of Lord Arnold McNair, that it was "reasonable" to expect international parties to live by the maxim allegans contraria non est audiendus, or "You cannot blow hot and cold."[2]

In the end, the estoppel argument was not hugely impactful for our moot case. Indeed, estoppel arguments often fail to hit the mark — it can be difficult to identify conduct by an opponent that is squarely inconsistent with its prior unambiguous representations or to meet the other criteria for estoppel. But experience teaches that, every now and again, an estoppel argument will stick. This was certainly the case in the recent Chagos arbitration.

1. The Principle of "Estoppel" in International Law

"Estoppel" is a doctrine that seeks to prevent parties from adopting inconsistent positions, particularly if the opposing party has relied upon such positions and would suffer prejudice if they were to change.[3] As noted already, the principal legal decision supporting the doctrine in the sphere of public international law, is Temple of Preah Vihear, a dispute between Thailand and Cambodia concerning sovereignty over a particular segment of their frontier (containing a historical temple). After it was shown that Thailand had actually agreed, through a prior boundary treaty with Cambodia, that the area in question was Cambodian territory,[4] the ICJ added that "[e]ven if there were any doubt" on the issue: "the Court would consider, in the light of the subsequent course of events, that Thailand is now precluded by her conduct from asserting that she did not accept it."[5] Thailand's' long acceptance of a 1908 map treating the area as Cambodian, and its enjoyment of "the benefit of a stable frontier" in the years since, meant that it was "immaterial" whether or not the map conformed with the treaty – it was "not now open to Thailand, while continuing to claim and enjoy the benefits of the settlement, to deny that she was ever a consenting party to it."[6]

The Court's opinion, as supplemented by the various individual ICJ judicial opinions, have since guided parties and tribunals in subsequent international cases in dealing with situations where one party has espoused inconsistent positions,[7] although often an estoppel-based argument has failed, on the given facts.[8] The doctrine has operated alongside other principles or maxims that, in the general sense, operate to prevent inconsistent or capricious state conduct – these include notion of good faith, pacta sunt servanda and acquiescence – indeed, there have been instances (notably in cases dealing with declarations concerning World Court jurisdiction) where the Court prohibited states from going back on their past positions on the basis of "good faith" considerations, without reference to Temple of Preah Vihear.[9]

Thus, while the principle of "estoppel" in Temple of Preah Vihear has been widely recognized and often pled by litigants, it has only rarely succeeded. But, every so often, instances still arise, as they did in the Permanent Court of Arbitration ("PCA") arbitration between Mauritius and the United Kingdom involving the Chagos Marine Protected Area, whose 2015 award[10] represents an important development in the field of estoppel in international law.

2. The Chagos Dispute
   1. The Colonial Position -- the Chagos Islands as a Historical Part of Mauritius

The Chagos dispute had its origins in the events surrounding Mauritius's independence.

Mauritius, of course, is an island state in the Indian ocean. Prior to 1968, when Mauritius became fully independent, it was administered by the United Kingdom – a situation that dated back to the Napoleonic Wars.[11]

To the north of Mauritius, there lies an archipelago of small atolls known as the "Chagos Archipelago."[12] During the time that Mauritius was subject to British control, the Chagos Archipelago was treated as a "Dependency of Mauritius," that is, administered by Britain as part of the same territorial unit as Mauritius.[13]

1. 2. The Pre-Independence Deal to Detach the Chagos Archepelago

Shortly prior to Mauritian independence, however, the British government formulated a plan to "separate the Chagos Archipelago from the remainder of the colony of Mauritius, and to retain the Archipelago under British control."[14] In the arbitration, Mauritius's counsel claimed that this "stemmed from a decision by the United Kingdom in the early 1960s to 'accommodate the United States' desire to use certain islands in the Indian Ocean for defence purposes.'"[15] In particular the Chagos island of Diego Garcia was thought to be an important military location. The local Mauritius leadership, however, expressed reluctance to agree to this proposal. Their acceptance of the "separation" of the Chagos Archipelago was secured only by high-level consultations with the UK Government at Lancaster House in London, attended by (on the Mauritian side) independence leader led by Sir Seewoosagur Ramgoolam and (on the British side), Secretary of State for the Colonies Anthony Greenwood, a cabinet minister in the Wilson Government.[16] This culminated in a "provisional agreement," referred to in the Chagos Award as the "1965 Agreement," in which the leaders agreed to the separation, on the basis that Greenwood would make certain recommendations to the UK Cabinet – essentially, to compensate Mauritians as well as the Chagos residents likely to be displaced by these arrangements, and to assure the Mauritian side that the Chagos Archepelago would eventually be returned to Mauritius.[17]

1. 3. The Lancaster House Undertakings

In the wake of this meeting, Mauritian leader Sir Seewoosagur Ramgoolam took steps to firm up the British side of the deal. He sent a handwritten letter to the Colonial Undersecretary, "setting out further conditions" considered important by the Mauritian side.[18] Among these were a request for "fishing rights" around the archipelago, and for "[a]ny mineral or oil discovered on or near islands to revert to the Mauritius Government."[19] These parts of the handwritten note were further discussed, and on September 23, 1965, they were incorporated into a formal meeting minute:

Summing up the discussion, the [UK] SECRETARY OF STATE asked whether he could inform his colleagues that Dr. [Sir Seewoosagur] Ramgoolam, Mr. Bissoondoyal and Mr. Mohamed were prepared to agree to the detachment of the Chagos Archipelago on the understanding that he would recommend to his colleagues the following:-

(i) negotiations for a defence agreement between Britain and Mauritius;

(ii) in the event of independence an understanding between the two governments that they would consult together in the event of a difficult internal security situation arising in Mauritius;

(iii) compensation totalling up to £3m. should be paid to the Mauritius Government over and above direct compensation to landowners and the cost of resettling others affected in the Chagos Islands;

(iv) the British Government would use their good offices with the United States Government in support of Mauritius' request for concessions over sugar imports and the supply of wheat and other commodities;

(v) that the British Government would do their best to persuade the American Government to use labour and materials from Mauritius for construction work in the islands;

(vi) the British Government would use their good offices with the U.S. Government to ensure that the following facilities in the Chagos Archipelago would remain available to the Mauritius Government as far as practicable:

(a) Navigational and Meteorological facilities;

(b) Fishing Rights;

(c) Use of Air Strip for emergency landing and for refuelling civil planes without disembarkation of passengers.

(vii) that if the need for the facilities on the islands disappeared the islands should be returned to Mauritius;

(viii) that the benefit of any minerals or oil discovered in or near the Chagos Archipelago should revert to the Mauritius Government.[20]

This statement elicited affirmative responses from the three representatives, subject only to their desire to consult further with colleagues in Mauritius and their hope that some of the terms (e.g., compensation) might be made more generous.[21] The proposals were then presented to the Mauritian representatives by the Mauritian Governor, who, on November 5, 1965 reported back the UK that the "Council of Ministers" (which included the local Mauritius leaders) "confirmed agreement to the detachment of Chagos Archepelago on the conditions enumerated [in the meeting minute]," on the "understanding" among other things, that the UK Government had agreed to those conditions.[22]

In the ensuing Chagos Award, the statements at paragraphs (i) through (viii) – assuring Mauritius of, among other things, fishing rights for Mauritians, the future benefit of discoveries of minerals or oil, and future sovereignty of the archipelago in the event the UK's defense needs were to expire – were referred to as the "Lancaster House Undertakings."

1. 4. The Separation of the Chagos Archipelago and
      Creation of the British Indian Ocean Territory

On November 8, 1965 (that is, three days after the Mauritian side had indicated acceptance of the Lancaster House Undertakings), the UK issued a proclamation detaching the Chagos Archipelago from the territory of Mauritius and established this area as a new overseas British territory, named the "British Indian Ocean Territory," or "BIOT"). [23] Mauritius attained independence two and a half years later, on March 12, 1968.[24]

Within BIOT, the island of Diego Garcia has remained a key American military base. The administration of the BIOT, however, has been highly controversial, due to the UK's forced removal from the area (and resettlement in Mauritius) of 1,500 and 1,750 Chagossian residents from the Chagos Islands, apparently in order to facilitate the Diego Garcia base. Their expulsion has led to multiple lawsuits, as well as payments by the UK of financial compensation, to the affected families.[25] As regards the waters surrounding the Chagos Archipelago/BIOT, there was also controversy.

1. 5. Britain's Early Claims to Sovereignty Over
      the Waters Surrounding the Chagos Islands

In the decades following independence, the UK government claimed sovereignty over the (then) 3-mile “territorial sea,” as well as a contiguous, 12-mile “fisheries zone,” around the Chagos Islands.[26] Nevertheless – and consistent with the Lancaster House Undertakings – the UK also enacted laws to allow Mauritians to fish in these waters.[27]

In this same period, the Mauritian government claimed sovereignty, with growing assertiveness, over both the islands and their surrounding maritime space, and refused to recognize the legitimacy of the BIOT.[28] The issue assumed considerable political importance within the Mauritius, but the Mauritian government did not, in these early decades, take aggressive legal steps to prosecute its claims.

Indeed, both Britain and Mauritius worked cooperatively, taking various positive steps regarding the Chagos Archipelago, while side-stepping the issue of sovereignty. For example, in 1984, the UK and Mauritius Governments established a British-Mauritian Fisheries Commission "to address the conservation of fish stocks," while at the same time stipulating that "neither the creation of the Commission nor any activity carried out pursuant to it would be understood to prejudice the Parties' respective positions regarding the Chagos Archipelago."[29]

1. 6. 1991: Britain's Declares a 200 NM "Fisheries Conservation
      and Management Zone" Around the Chagos Archipelago

In the early 1990s, the UK took things a step further. By now, the UK (as well as Mauritius) had signed the 1982 Law of the Sea Convention ("LOSC"), which was on the verge of coming into force. By its terms, the LOSC permits coastal state to exercise sole economic control over a contiguous 200 nautical mile zone around its coast, known as the Exclusive Economic Zone, or "EEZ."[30]

Although it did not formally claim an EEZ around the BIOT, in 1991, the UK proclaimed a 200 nautical mile "Fisheries Conservation and Management Zone," or "FCMZ," around the BIOT.[31] Mauritius immediately responded "by reiterating its claim to sovereignty over the Chagos Archipelago."[32]

1. 7. 2003: The UK Converts the FCMZ to a "Environmental
      Protection and Preservation Zone"

Things escalated further in the 2000s. In August 2003, seeking to ramp up environmental protection in the BIOT, the UK redesignated the FCMZ as a "Environmental Protection and Preservation Zone" ("EPPZ") although it sought to mollify Mauritian sovereignty concerns by stating that that this "was not a full exclusive economic zone for all purposes."[33] When the UK deposited the coordinates of the EPPZ with the United Nations Secretary General, Mauritius lodged a formal protest to the Secretary General.[34]

1. 8. 2009: the Brown Government Declares the Chagos
      Archipelago Waters to be a "Marine Protected Area"

Then, in 2009, the government of Prime Minister Gordon Brown, prompted by environmental concerns, began to formulate a policy to turn the waters of the BIOT, a "giant marine park," known as the "Marine Protected Area," in which fishing would be banned.[35] It stated concern was to preserve "biodiversity" in the area and to preserve the "beauty" of the area.[36] Between July 2009 and March 2010, the UK Government had a series discussions with the Mauritian government about this initiative; for its part, while accepting in principle the desirability of environmental protection, the Mauritius government was reluctant to accede to any proposal that would imply British sovereignty over the waters of the BIOT.[37]

In March 2010, the UK government's representatives stated that no decision on a Marine Protected Area had yet been taken, and that "'[l]ike Mauritius, the UK is keen to continue these bilateral talks as there are many other things we can discuss with regards to BIOT.'"[38] Only a few weeks later, however, in late March 2010, UK Foreign Secretary David Miliband decided to go ahead with the marine protection area.[39]

The reasons for rushing forward with this decision are not wholly clear, but it seems very likely, in hindsight, that it was dictated by internal political events: Prime Minister Brown declared a general election in Britain barely a week after this decision, on April 6, 2010. [40] In the arbitration, the UK later admitted:

there was an election due at the beginning of May, which was a little over four weeks later. In the British system of government, when an election is called, essentially government stops. No new policies can be introduced. So, either Mr. Miliband took his decision on 1 April – which is the last possible date he could do so before the election – or he could leave the decision for the incoming government four weeks later.[41]

Evidently, Foreign Secretary Miliband was unwilling to place this decision in the hands of the next government (assuming the Brown Government lost the election, which, as events transpired, is what happened.)

In any case, on April 1, 2020, on Mr. Miliband's instruction, the BIOT Commissioner issued Proclamation No. 1 of 2010, formally establishing an MPA.[42] Mauritius responded in strong terms, stating that it "[did] not recognize the so-called British Indian Ocean Territory and that the Chagos Archipelago, including Diego Garcia, forms an integral part of the sovereign territory of Mauritius both under our national law and international law," and that it "[would] not recognize the existence of the marine protected area . . . and [would] look into legal and other options that [were] now open to it." [43]

For Mauritius, this was the final straw. In December 2010, it instituted arbitral proceedings (under the LOSC's dispute resolution procedures) aiming to vindicate, one and for all, its claims to the Chagos Archipelago.

3. The Chagos Arbitration and Award
   1. The Constitution of the Chagos Tribunal and Procedural Steps

During 2010 and 2011, an arbitral tribunal was constituted in accordance with the LOSC's dispute resolution procedures, as laid down in LOSC Annex VII. Under these provisions, an arbitral tribunal is to consist of five arbitrators – with two co-arbitrators appointed by the parties, and three neutral arbitrators, appointed either by agreement of the parties or, in default of such agreement, by the President of the International Tribunal for the Law of the Sea ("ITLOS").[44]

Pursuant to these provisions, the two sides appointed co-arbitrators: Mauritius appointed Judge Rüdiger Wolfrum (himself an ITLOS judge) and the UK appointed Sir Christopher Greenwood, then an ICJ judge, and a UK national.[45] The two sides being unable to agree upon the three neutral arbitrators, this task was referred to the President of ITLOS, who appointed ITLOS Judges James Kateka (of Tanzania), Albert Hoffmann (of South Africa), and Professor Ivan Shearer of Australia the latter of whom was also appointed as President of the Tribunal.[46] By agreement, the arbitration was administered under the auspices of the PCA.[47]

Over the next two years, written briefing was received from both sides, leading to a hearing in Istanbul, Turkey in mid-2014.[48] An award was issued in March 2015. As it turned out, estoppel played a major role in the final determination.

1. 2. The Tribunal Rejects Three of Mauritius's Four Claims

At the outset of the Award, the Tribunal first had to grapple with the limitations of LOSC compulsory arbitration. Under LOSC's arbitration provisions, member states are only entitled to seek arbitration over "any dispute concerning the interpretation or application of [the LOSC]."[49] Seeking to fit within these parameters, Mauritius asserted four claims, all purporting to establish a maritime dispute under the LOSC:

(1) Mauritius argued that the MPA was invalid because the UK was not the "coastal State" of the Chagos Islands, as that term was understood under the LOSC;

(2) Mauritius argued that the UK's prior commitments to Mauritius meant it was not "entitled" to declare an MPA or other maritime zones around the Chagos Islands;

(3) Mauritius sought a declaration that it could lay claims to the continental shelf surrounding the Chagos Archipelago with the relevant UN authorities; and

(4) Mauritius claimed that the UK's declaration of "MPA" was "incompatible with the substantive and procedural obligations of the United Kingdom under the [LOSC]," including articles 2, 56 and 194, as well as the LOSC's implementing arrangements regarding fisheries management. [50]

The UK had challenged its jurisdiction over all of these Mauritius claims, arguing that Mauritius was essentially seeking to challenge the UK's sovereignty over a land area (the Chagos Archipelago), whereas the LOSC itself does not regulate title to land areas, much less permit arbitration over such matters.[51] As such, it had argued, Mauritius's claims were beyond the tribunals' proper jurisdiction.

The Tribunal upheld these jurisdictional challenges as regards Mauritius's first two claims. By majority, it found that Mauritius's first claim (which sought to deny the UK's status as a "coastal state" with respect to the Chagos Archipelago) was outside the proper jurisdiction of an LOSC tribunal, and that it would not read the LOSC's provisions as "a warrant to assume jurisdiction over matters of land sovereignty."[52] By majority, it also reached the same conclusion with respect to the second claim, holding that its "predominant" feature was the parties' "underlying dispute regarding sovereignty over the Archipelago."[53] Mauritius's third claim (seeking to preserve Mauritius's future ability to assert rights over the Chagos Archipelago's continental shelf) also was dismissed, albeit on different grounds: the Tribunal unanimously concluded that the UK had not actually sought to interfere with such rights, meaning that there was no genuine legal dispute capable of adjudication at that time.[54]

1. 3. The Fourth Claim Survives Dismissal – Prompting an Estoppel Analysis

Mauritius's case, thus reduced, hinged solely on its fourth claim, which contended that the declaration of the MPA itself was "incompatible" with its obligations under the LOSC. The Tribunal upheld jurisdiction over this claim alone.[55]

In this claim, Mauritius claimed, first, that the declaration conflicted with the express commitments to Mauritian leaders made by the UK Government as 1965 as part of the "Lancaster House Undertakings," in which the government assured Mauritius it would have certain rights in the maritime spaces around the Chagos Archipelago.[56] This, in turn, led to two alleged breaches of the LOSC: Article 2(3), requiring that a state's "'sovereignty over the territorial sea'" be "'exercised subject to [the LOSC] and to other rules of international law,'" and Article 56(2), providing that "'[i]n exercising its rights and performing its duties under [the LOSC] in the exclusive economic zone, the coastal State shall have due regard to the rights and duties of other States and shall act in a manner compatible with the provisions of [the LOSC].'"[57]

The finite – indeed restrictive – parameters of this fourth claim should be self-evident. In rejecting the first the claims, the Tribunal had definitively ruled out any direct challenge to the UK's sovereignty over the Chagos Archipelago. To prevail on its fourth claim, therefore, Mauritius had to prove that, even assuming the UK owned the Chagos Archipelago and was the "coastal state," it still had no right to exercise unfettered maritime sovereignty over the surrounding territorial sea and exclusive economic zone – even though the LOSC presumptively granted such sovereignty to a coastal state. Moreover, Mauritius had no past treaty or maritime boundary agreement with the UK – and thus had none of the traditional means of questioning another state's sovereignty. All was riding on the 1965 Lancaster House Undertakings – pronouncements that were unilateral in nature, and issued three years before Mauritius acquired independence.

1. 4. The Tribunal Finds the Lancaster House Undertakings to
      be a "Firm Commitment," Cognizable Under International Law

At the outset, the Tribunal made two key findings about the Lancaster House Undertakings. First, it noted that, in assuring Mauritian independence leaders that Mauritians would have the rights to use the waters around the Chagos Archipelago, the United Kingdom's ministers were not mere blandishments. Rather, read in context, "the United Kingdom intended [nothing] less than a firm commitment that would shape its relations with Mauritius following independence."[58] As the Tribunal saw it:

Independence in the near future was expected, and the commitments made by United Kingdom were not aimed at the narrow window of time between detachment and independence, but at future relations between the United Kingdom and an independent Mauritius. Moreover, the United Kingdom itself described its commitment in the language of obligation. In requesting that the conditions be presented to the Mauritian side, the Governor of Mauritius was asked on 6 October 1965, to secure Mauritian agreement to detachment "on the conditions" set out in the Lancaster House Meeting. To the Tribunal, these are not the words of a voluntary intent to assist Mauritius to the extent politically feasible, but of an offer made on the basis of an intent to be bound.[59]

The Tribunal further noted that the Lancaster House Undertakings were the basis upon which Mauritian leaders entered into an agreement (referred to as the "1965 Agreement") for the detachment of the archipelago. And although, as the UK had observed, the agreement had no "international law" status at the time it was made (it being a mere agreement between a "non-self-governing territory" and the UK), [60] things changed when Mauritius acquired independence:

425. Had Mauritius remained part of the British Empire, the status of the 1965 Agreement would have remained a matter of British constitutional law. The independence of Mauritius in 1968, however, had the effect of elevating the package deal reached with the Mauritian Ministers to the international plane and of transforming the commitments made in 1965 into an international agreement. In return for the detachment of the Chagos Archipelago, the United Kingdom made a series of commitments regarding its future relations with Mauritius. When Mauritius became independent and the United Kingdom retained the Chagos Archipelago, the Parties fulfilled the conditions necessary to give effect to the 1965 Agreement and, by their conduct, reaffirmed its application between them.

. . .

428. Accordingly, the Tribunal concludes that, upon Mauritian independence, the 1965 Agreement became a matter of international law between the Parties. Moreover, since independence the United Kingdom has repeated and reaffirmed the Lancaster House Undertakings on multiple occasions. This repetition continued after Mauritius began proactively to assert its sovereignty claim in the 1980s, and even after such a claim was enshrined in the Constitution of Mauritius in 1991.

. . .

434. All told, the Tribunal is faced with undertakings given as part of an agreement concluded in 1965 between the United Kingdom and one of its colonies, that became a matter of international law upon the independence of Mauritius, and that were reaffirmed in correspondence between the Parties in the decades since independence.[61]

1. 5. The Tribunal Reaffirms that Estoppel is a Rule of International Law

Against that background, the Tribunal held that this was an appropriate setting for the application of estoppel – the principle that "in the words of Lord McNair, 'that international jurisprudence has a place for some recognition of the principle that a State cannot blow hot and cold—allegans contraria non audiendus est.'"[62] In that respect, the Tribunal endorsed the formulation of the principle as set forth in Sir Percy Spender's opinion in Temple of Preah Vihear. already quoted above,[63] as well as numerous subsequent ICJ decisions that "added definition to the scope of the principle, [64] which it summarized as follows:

estoppel may be invoked where (a) a State has made clear and consistent representations, by word, conduct, or silence; (b) such representations were made through an agent authorized to speak for the State with respect to the matter in question; (c) the State invoking estoppel was induced by such representations to act to its detriment, to suffer a prejudice, or to convey a benefit upon the representing State; and (d) such reliance was legitimate, as the representation was one on which that State was entitled to rely. [65]

1. 6. The UK is Held To be Legally Bound, Through Estoppel,
      to Honor the Lancaster House Undertakings

Applying these principles to the case at hand, the Tribunal found the first two elements to be "readily fulfilled": the UK had made "repeated representations in respect of all three undertakings over the course of over 40 years."[66] These, furthermore, were plainly authorized by the state.[67] The Tribunal added that the UK's conduct – its "unvaried practice of permitting Mauritian fishing in the waters of the Archipelago" – also "constituted a representation by conduct that such fishing rights would be continued, not necessarily unconditionally, but at least in the absence of an exceptional change of circumstances."[68]

The Tribunal also found that Mauritius's conduct (particularly its silence in the face of the UK asserting sovereignty over the archipelago) showed that it had relied to its detriment on the Lancaster House Undertakings:

Mauritius' entire course of conduct with respect to the Chagos Archipelago was undertaken in reliance on the full package of undertakings given at Lancaster House. From independence until at least 1980, Mauritius was silent as to the legitimacy of detachment. . . . . The Tribunal considers this initial silence, and Mauritius' comparatively restrained assertion of its sovereignty claim thereafter, to have been a result of the undertakings given by the United Kingdom.[69]

The Tribunal further noted that this silence was maintained even when "sentiments in favour of decolonization were still running high," and that, had the UK not made the initial package of undertakings, Mauritius "would have asserted its claim to the Archipelago earlier and more directly" (as eventually occurred in 2009-10, when the MPA was declared).[70] Thus:

Accordingly, the Tribunal concludes that Mauritius relied, both specifically and generally, on the package of undertakings given and reaffirmed by the United Kingdom. In so doing, Mauritius forewent the opportunity of pressing its sovereignty claim in the initial years following independence, forewent the United Kingdom's offer to conclude a treaty formalizing the commitment to eventually return the archipelago, and conveyed a benefit on the United Kingdom through the cooperation on other matters that the Tribunal believes would otherwise have been withheld.[71]

Finally, the Tribunal found that as a matter of law, Mauritius was entitled to rely upon the Lancaster House Undertakings because they were "were consistently reiterated after independence in terms which were capable of suggesting a legally binding commitment and which were clearly understood in such a way."[72] It thus concluded that

after its independence in 1968, Mauritius was entitled to and did rely upon the Lancaster House Undertakings to (a) return the Chagos Archipelago to Mauritius when no longer needed for defence purposes; (b) preserve the benefit of any minerals or oil discovered in or near the Chagos Archipelago for the Mauritius Government; and (c) ensure that fishing rights in the Chagos Archipelago would remain available to the Mauritius Government as far as practicable. The Tribunal, therefore, holds that the United Kingdom is estopped from denying the binding effect of these commitments, which the Tribunal will treat as binding on the United Kingdom in view of their repeated reaffirmation after 1968.[73]

1. 7. The UK's Declaration of an MPA Breaches the Lancaster House Undertakings – and, as a Consequence, Breaches LOSC Articles 2(3) and 56(2)

Having found that the UK was legally bound by the Lancaster House Undertakings, the Tribunal returned to the provisions of the LOSC at issue. These, it held, meant that "Article 2(3) [of the LOSC] requires the United Kingdom to exercise good faith with respect to Mauritius' rights in the territorial sea," and that Article 56(2) "requires the United Kingdom to have due regard for Mauritius' rights in the exclusive economic zone."[74] Given that the declaration of an MPA would plainly impact those rights, these obligations "entail[ed] at least, both consultation and a balancing exercise with its own rights and interests."[75]

In the case of the MPA, the Tribunal found, the UK's conduct fell short both of the obligation to consult and the obligation to "balance" interests. In the Tribunal's view, "[n]ot only did the United Kingdom proceed on the flawed basis that Mauritius had no fishing rights in the territorial sea of the Chagos Archipelago, it presumed to conclude—without ever confirming with Mauritius—that the MPA was in Mauritius' interest."[76] These failings, it held, meant that the UK "breached Articles 2(3) and 56(2) and therefore . . . that the proclamation of the MPA was incompatible with the [LOSC]."[77] The Tribunal found that the UK's conduct also violated another provision, Article 194(4), which requires that, in enacting anti-pollution measures, "States shall refrain from unjustifiable interference with activities carried out by other States in the exercise of their rights and in pursuance of their duties in conformity with this Convention."[78]

The Tribunal, in closing, stressed that it was not commenting on the merits of the MPA as an environmental measure, and that it expected that this issue could be the subject of further negotiation among the parties.[79] This, however, may be been an unduly modest statement – the true effect of the Award was to reaffirm Mauritius's long-term interests in the BIOT/Chagos Archipelago and surrounding waters. The Award will surely buttress Mauritius's long-term claims to these regions.

* * *

From a practitioner's standpoint, the Chagos Award surely also represents the most notable invocation of estoppel in international law since the Temple of Preah Vihear case. Albeit that the two cases are separated by more than a half-century, and arose in very different circumstances, and yet they now represent a consistent line of jurisprudence on this vital issue.

Which brings this writer back to his Jessup mooting anecdote. The professor/coach who, decades ago, urged my team to look at Temple and McNair in preparation for our argument was none other than Professor Ivan Shearer – the chair of the Chagos arbitration tribunal. Ivan Shearer was a brilliant and generous teacher, as well as a leading world authority on the law of the sea, and it was thus no surprise that, in later years, he became a brilliant and wise arbitrator, both in this and several other important maritime disputes of the last two decades. Sadly, Professor Shearer died in 2019, aged 81, and was greatly mourned by colleagues and former students, including my contemporaries.

As should be evident, this is not an obituary,[80] but rather a brief look at one of Professor Shearer's final (and finest) accomplishments as arbitrator. The Chagos Award is a tribute to his scholarship and rigor, as well as his intrinsic sense of fairness. Hopefully, for both the British and Mauritian people, it will have a beneficial impact in the decades ahead.