The Claimants argued that an Act of Parliament was required to authorise the triggering of Article 50, and the Supreme Court has ruled (by a majority of 8 to 3) that they were right.  The Court confirmed that withdrawal from the EU would result in fundamental change to the UK's constitutional arrangements, by cutting off the source of EU law and by removing existing domestic rights of UK residents and, as such, withdrawal can only be triggered with Parliamentary approval.

In practical terms, yesterday's judgment appears unlikely to affect the Prime Minister's timetable for withdrawal, or her Government's objectives for Brexit. However, it does mean that the next step in the process of the UK's withdrawal from the EU is for Parliament to take. The Supreme Court has made clear that Article 50 can only be authorised by way of an Act of Parliament. It did not prescribe what form that Act must take, saying that this is entirely a matter for Parliament (although it noted that the Act need not be lengthy).

Ministers have been managing expectations for some time, and will no doubt be ready to react. The Government will be looking to get a Bill through Parliament quickly. Precedent suggests that it will be able to do that (indeed, following the events of 9/11, the Anti-Terrorism, Crime and Security Act 2001 passed through Parliament in only 32 days). There is a real risk, however, that the Government may have to make concessions on parliamentary involvement in the process along the way. Already, the SNP has said that its MPs will table amendments to the Bill, and some Labour and Liberal Democrat MPs have announced that they will vote against the Bill.  

It is significant that the Court has confirmed that the devolved legislatures in Scotland, Northern Ireland and Wales do not have a veto on Brexit and that the Government has no legal obligation to consult them. This could have caused material delay for the Government's plans, not least given the forthcoming Stormont elections.  However, the Court did emphasise that the Sewel Convention (regarding Westminster consultation with and consent from the devolved legislatures) acts as a political constraint that plays an important role in the operation of the UK constitution. We can expect the devolved legislatures to assert the significance of this statement in the coming weeks.

More broadly, this ruling addresses a number of fundamental constitutional issues that rarely fall to be considered by the courts, including the interplay between the Government's executive powers in international law and legislative sovereignty, the powers of the Court to supervise the exercise of prerogative powers particularly where existing law or rights are affected, and the fundamentally political, not legal, nature of the Sewel Convention on consultation of devolved legislatures.  However, the Court was at pains to say that its judgment reflects well-established constitutional principles.

One issue that the Court left unresolved was the question of whether an Article 50 notice could be revoked by the UK after the Brexit process has been triggered.  The Government expressly resisted this issue being ruled on even though the Divisional Court noted that, if it could be revoked, the claim would be "blown out of the water". The Supreme Court decided to make its ruling on the basis of an assumption that Article 50 was irrevocable.  Ultimately, whether this assumption is correct can only be determined by the Court of Justice of the European Union, and other claimants have already come forward seeking to have that question decided.