On 24 January 2017, the UK Supreme Court held that the UK Government needs the UK Parliament's approval before a notice under Article 50 of the Treaty on the EU, notifying of the UK's intention to withdraw from the EU and commencing the negotiation process for that withdrawal, may be provided to the European Council. The Court also ruled that the UK Parliament (sitting in Westminster, London) does not need to consult or seek the consent of the devolved legislatures in Scotland, Northern Ireland or Wales to trigger the UK's withdrawal from the EU under Article 50.
The UK Government had argued before the Supreme Court that it had the authority to issue an Article 50 notice without any statutory approval from the UK Parliament. That argument was based on the royal prerogative powers, which continue to exist even today under the largely unwritten UK Constitution. Those powers include the power to declare wars, issue pardons and -- most importantly for this case -- the power to enter as well as withdraw from international treaties with other states.
The Court rejected the UK Government's argument on the basis of two central reasons. First, a withdrawal from the EU would fundamentally alter the UK's constitutional design by removing from it one of the UK's current sources of law; that is, the EU's law-making institutions. The connection between those sources and the UK's legal system was established under a statute of the UK Parliament -- the European Communities Act 1972 -- and it follows that a statute is needed to remove that connection. Second, such a withdrawal would have the consequence of taking away domestic rights of UK residents, which rights have arisen pursuant to EU law -- and which rights were also given effect to in the UK under the 1972 statute of the UK Parliament. So, while the UK Government retains the prerogative power to enter and withdraw from international treaties, that power is limited: it does not extend to the removal of sources of UK law or the removal of existing rights of UK residents. Any attempts to exercise such a power by issuing an Article 50 notice without the UK Parliament's approval would have been unconstitutional.
The Supreme Court is the court of highest instance in the UK, so no other judge or court in the UK can reverse this decision. Moreover, given that the arguments before the Supreme Court were limited to questions concerning UK's constitutional law and did not involve questions of EU law, no party to the proceedings can appeal to the European Court of Justice. So, the outcome of the Supreme Court's decision is final: the UK Parliament sitting in Westminster has the power to stop the UK Government from issuing an Article 50 notice, but there is no scope for the devolved legislatures to intervene in the triggering of Article 50.
As regards what will happen next, that is less certain. In a statement released by the UK Prime Minister's Office shortly after the Supreme Court's judgment had been published, Theresa May emphasised that her intention remains to trigger Article 50 negotiations by the end of March 2017. In view of the Supreme Court's decision, that intention may result in the UK Government setting out its plan for withdrawing from the EU -- and possibly the UK's alternative arrangements with the EU following any final withdrawal from it -- in a lot more detail in the coming weeks than until now, before a vote on an Article 50 notice takes place in the Westminster Parliament. It may also mean that if the May Government's plans are thwarted by the UK Parliament with the effect that no Article 50 notice is issued by the end of March 2017, an early election in the UK may be called this year to give the electorate a chance to vote on the type of deal with the EU that the people of Britain want.