The Supreme Court’s Brexit Judgment

Who has the legal power to start the process to take the UK out of the EU  (or ‘trigger Article 50’ of the Treaty of Lisbon) – the Government or Parliament? That was the main question decided in the Brexit judgment of the UK Supreme Court.

The Government had argued that it could do so alone using its ‘prerogative’ (executive) powers. Applicants in the English case of Miller, and in the Northern Irish case of Agnew and others had argued that an Act of Parliament was required.

The Supreme Court decided by an 8-3 majority that the Government could not trigger Article 50 without an Act of Parliament.

In addition to that core question there were other arguments raised in the Northern Irish cases of Agnew and McCord. These included the question as to whether the UK Parliament needed to seek the consent of the NI Assembly to any Act approving the triggering of Article 50 (a question affecting all the devolved nations), and questions more specifically about Section 75’s equality obligations, and whether the consent of the people of Northern Ireland was required.

Majority opinion

The 8 judge majority opinion is unusual in that all 8 judges have written it. This joint authorship signals to potential critics in the media or political world, that the majority regard this as a clear, legal position, and there is no scope to quibble about legal niceties much less cast doubt on its authority.

This is reinforced by the language used to describe the Government’s arguments  – one argument ‘plainly does not apply’ (para 84); the Government’s argument would provide it with a ‘far-reaching and anomalous’ right (para 87); the Government’s arguments are ‘improbable’ (para 91), ‘bold’ (para 92), ‘potentially controversial’ (para 92), implies ‘incongruous’ results (para 132).

The majority judgment is broadly in four parts. There is an introductory section on the EU and on prerogative powers  (13-59). Once the introduction is covered there are three key points of discussion. The core of the judgment considers whether the prerogative can be used to trigger Article 50 or is this precluded by the European Communities Act 1972 (60-125). This is followed by a short discussion of the Northern Ireland issues (126-135) and a lengthier discussion of the devolution question (136-151).


The Government’s prerogative powers must yield to an Act of Parliament; all agree on that. The disagreement in this case between the Government and the applicants is whether any Act rules out the use of prerogative powers to trigger Article 50. There is no Act which explicitly says this in clear words. Does any Act, properly interpreted, imply this? According to the Supreme Court the answer is yes: the European Communities Act 1972 (ECA).

There are two closely related reasons for this. First the European Communities Act 1972 (ECA) created a major constitutional innovation introducing a new source of law (EU law) into the legal system. This cannot be undone by anything less than another Act of Parliament (paras 80-82). Connected with that, if the prerogative were to be used to trigger Article 50 it would inevitably change rights held by virtue of EU law (para 83). This again cannot be done by mere prerogative.

Northern Ireland

The discussion of specific Northern Ireland questions (apart from the broader devolution discussion) is brief.

The Supreme Court rejects the argument that the provision for a referendum on the future of Northern Ireland in section 1 of the Northern Ireland Act meant Article 50 could not be triggered without the consent of the people of Northern Ireland. According to the Court that ‘important provision’ only relates to the decision whether to remain part of the UK or to unite with Ireland and does not regulate ‘any other change in the constitutional status’ (para 135).

The Supreme Court does not decide several NI questions because they are ‘superceded’ by the decision about the European Communities Act, though it does comment on them.

The Supreme Court considers  whether the Northern Ireland Act precludes the use of the prerogative to trigger Article 50 without an Act of Parliament. The Court strongly hints that the NIA does preclude this (paras 131-132) but ultimately concludes ‘it is not necessary to reach a definitive view’ (para 132).

On the argument that the  Section 75 equality obligations apply to the Secretary of State’s role in Brexit: the Supreme Court says this question is ‘superseded’ but nevertheless indicates a view that Section 75 does not apply because giving notice to leave the EU ‘is not a function carried out by the Secretary of State for Northern Ireland in relation to Northern Ireland within the meaning of section 75’ (para 133).

The brief discussion on Northern Ireland issues hardly does justice to the lengthy and detailed arguments in the Agnew and McCord cases. It may reflect the pragmatic approach that having decided bulk of issues in relation to the ECA argument, the Court did not need to decide these issues. Or perhaps judges in the majority have more nuanced  views on these issues but the need for agreement on an 8 judge decision lead to this brief discussion of the Northern Irish issues.


Turning to the third major issue, the devolution discussion: if an Act of Parliament is needed to trigger Article 50, then is there a legal obligation to get the consent of the Northern Ireland Assembly for that Act?

There is a convention on this (often called the Sewel or legislative consent convention); a convention is a political custom or tradition which is treated as politically or morally obligatory, but not legally enforceable. According to the convention, the Westminster Parliament will not normally legislate on devolved matters without the consent of the relevant devolved Parliament or Assembly. This is the narrow view of this convention.  On a broader view, the convention also applies when the Westminster Parliament alters the competences of the devolved Parliament or Assembly. Recently, the Scotland Act 2016 gives recognition to the narrow version of this convention in relation to Scotland.

In the Brexit judgment, the Supreme Court majority concludes that the Sewel convention is an important part of the political system but remains a convention and therefore cannot be enforced in the courts as a legal requirement.  The language in the Scotland Act does not change its status as a convention. Therefore, there is no legal enforcement of the obligation to seek consent from the devolved assemblies. This has caused consternation in some quarters at least in Scotland and Northern Ireland – the Scottish First Minister highlighting the implication that the ‘guarantee’ about the convention in the Scotland Act is ‘not worth the paper it is written on’.

That is the legal position; what is possible, desirable or required as a political matter is not decided by the Supreme Court. The Supreme Court judgment does not preclude the argument that consent is required by the convention, but that is an argument that has to be settled in the political not the judicial arena.

There are still very strong arguments that means have to be found in the political constitution to take account of the interests of the devolved nations and in particular the special features of the Belfast / Good Friday Agreement. There are mechanisms to do this within Parliament, mechanisms created as part of the devolution settlement and of course the North-South and East-West institutions created pursuant to the Agreement. We now need to see whether the political constitution and political leaders can rise to this challenge.

I am grateful to Daniel Holder, CAJ, and Colin Harvey, QUB, for comments on an earlier draft. This posts builds on comments in a post on the  Just Security blog at