R (Miller) v Secretary of State for Exiting the European Union [2017] UKSC 5

Following the 2016 referendum, the UK Government intended to notify the European Council of withdrawal under Article 50 TEU using the royal prerogative. Gina Miller and other claimants argued that ministers needed an Act of Parliament because withdrawal would change domestic law and remove rights given effect through the European Communities Act 1972. The Divisional Court accepted that argument, and the Government appealed. Related references raised Northern Ireland and devolution questions. The Supreme Court proceeded on the parties' shared assumption that an Article 50 notification could not be revoked, without deciding that EU-law question itself.

Could ministers use prerogative powers to give Article 50 notice without legislation, and did the devolution arrangements require consent from the devolved institutions before withdrawal?

By a majority of eight to three, the Supreme Court held that an Act of Parliament was required before ministers could give notice. Although treaty relations ordinarily fall within the prerogative, ministers cannot use it to change domestic law or remove statutory rights. The 1972 Act had made EU law a source of domestic law; withdrawing from that system involved a fundamental change requiring parliamentary authority. The referendum legislation did not itself confer the necessary power. The Court also held that the Sewel Convention remained political rather than judicially enforceable, and the devolved legislatures had no legal veto. The dissenters' different interpretation of the 1972 Act did not form the majority rule.

Prerogative powers cannot be used to alter domestic law or remove rights created through legislation without parliamentary authorisation. In Miller, that principle required legislation before Article 50 notification. Recognition of a constitutional convention in statute did not, in this context, convert it into a judicially enforceable rule.

Miller is a leading illustration of parliamentary sovereignty, limits on executive power and the distinction between law and convention. It should be kept separate from the later Miller/Cherry litigation concerning prorogation. The Court's assumption about irrevocability was subsequently displaced by Wightman, which recognised unilateral revocation under specified conditions. That development concerns Article 50's EU meaning and should not be mistaken for the Supreme Court having decided the point or for a reversal of its domestic constitutional holding.