10/12/2025
Written by: Helen Mountfield KC
In Miller No 1, or the Article 50 case as it is often known, an 11-judge Supreme Court decided, by a 8-3 majority, that Parliamentary authority was required to permit the Prime Minister to invoke Article 50 of the Treaty on European Union (TEU) following the Brexit referendum of 24 June 2016, and thereby to notify the EU of Britain’s intention to leave.
The case was politically charged, and made prime-time national and international news (and has contributed to ongoing debates about the proper interpretation of the rule of law and alleged judicial overreach).
It was super-expedited – leapfrogging straight from a 3-judge Divisional Court (the infamous ‘enemies of the people’) which heard the case over three days in October 2016 and delivered its judgment in early November, to a four-day, nine judge Supreme Court hearing in the December of the same year (and a judgment on 24 January 2017).
And it was a true constitutional landmark: University classes in constitutional law now start with “Miller One” as the ‘101’ for understanding the workings of the UK Constitution. The case generated 111 short-form articles on the UK Constitutional Law Association blog alone, many by some of the most pre-eminent constitutional scholars, and many of which were cited before and discussed by the Supreme Court.
But for the Matrix barristers involved, the result was never a foregone conclusion.
The primary legal question in the case was whether the government could use the executive’s foreign prerogative power to notify the Commission of the UK’s intention to leave via the mechanism in Article 50 TEU without first obtaining statutory authority to do so from Parliament. Some thought the executive power over international affairs was so clear and so long-standing that this was obviously a matter for the executive. But there are equally well-established legal conventions which go back to the Bill of Right and the English Civil War, namely that Parliament is sovereign, and that the executive cannot act in a way which nullifies or alters rights granted by Parliament or other fundamental rights without express statutory authority. These two principles appeared to be in tension.
The majority of the Supreme Court agreed with the unanimous three-judge Divisional Court that the foreign relations prerogative did not extend to extinguishing rights conferred by or through Parliamentary authority – including rights created by the EU legal order which, by virtue of the European Communities Act 1972 (and subsequent statutes granting extensions to EU authority), Parliament had legislated to join. And since rights granted under EU law were conferred by Parliament’s decision, only Parliament could consent to those rights being removed. So the executive could not undo the TEU in the UK unless Parliament had authorised this by statute, which it had not.
The Supreme Court majority reinforced its reasoning on the basis of a principle derived from a number of seventeenth and eighteenth century precedents, researched and drawn to their attention by the Pigney team, and clearly articulated by Lord Oliver in the Tin Council case, JH Rayner(Mincing Lane) v Department for Trade and Industry [1990] 2 AC 418 at 499E–500D:
“…as a matter of the constitutional law of the United Kingdom, the Royal Prerogative, whilst it embraces the making of treaties, does not extend to altering the law or conferring rights upon individuals or depriving individuals of rights which they enjoy in domestic law without the intervention of Parliament.”
However, three judges, Lords Reed, Carnwath and Hughes, dissented, and would have held that the royal prerogative in foreign affairs was wide enough to cover the power to withdraw from treaties, even a treaty like the TEU which created its own legal order, without needing further parliamentary permission.
A secondary argument was that the executive could not unilaterally invoke Article 50 because of rights created by the devolution settlement, and the effect of the Sewel Convention, which says that Westminster will normally seek consent from the devolved legislatures in Scotland, Wales and Northern Ireland when legislating on devolved matters. But the Supreme Court unanimously rejected that argument, holding that this was merely a political convention, not a legally enforceable rule.
The practical effect of Miller No 1 was minimal: the European Union (Notification of Withdrawal) Act 2017 was passed by Parliament on 16 March 2017, less than two months after the Supreme Court decision. But the constitutional consequences were highly significant, emphasising the principles of parliamentary sovereignty and the rule of law, and the reinforcing the limits on executive prerogative power – even in foreign affairs.
Matrix members
Rhodri Thompson KC for Gina Miller, Jessica Simor KC for Dier Dos Santos, and Helen Mountfield KC for Pigney and others.