The UK Supreme Court decision in the Brexit litigation on Tuesday is the most important judgment that the Court has delivered in its short history. Its 283 paragraphs will provide much material for academic, policy, and political debate for years to come.

The Supreme Court decided that the formal process to leave the European Union requires an Act of Parliament, and cannot be initiated by executive (“prerogative”) powers alone. This shifts the focus of the Brexit process to the UK Parliament. For strong supporters of Brexit this may be of concern, because prior to the Brexit referendum, it was clear there was no Parliamentary majority in favor of Brexit.

The UK’s constitutional context

As part of the UK’s devolution system, Scotland has a Parliament, Northern Ireland an Assembly and Wales a National Assembly with varying degrees of powers over “devolved” matters (hence “asymmetrical devolution”). The devolution system is importantly different from the US federal model: there is no written constitution which gives enforceable legal protection to the devolved nations. England, with more than 80% of the UK’s population, has no devolved assembly. Rather the UK Parliament makes laws for matters that only affect England as well as laws that affect the whole of the UK.

Northern Ireland’s system is more distinctive still. It is not just an example of UK devolution. Its constitutional arrangements are based on a peace agreement, the Belfast or Good Friday Agreement, which includes an international treaty between the UK and Ireland. This Agreement was endorsed in a referendum held on both sides of the Irish border, North and South. It includes powesharing institutions within Northern Ireland, special institutions which recognize the relationships between North and South (or Ireland), and which recognizes the relationships between all of the different institutions in the UK and Ireland (so-called East-West relationships between Ireland and Britain). The Agreement includes important human rights and equality guarantees. All these features make it a very different constitutional framework from the traditional UK model. 

The Referendum of June 23, 2016

As a brief reminder of the decision’s context: on June 23, 2016 the United Kingdom voted to leave the European Union by 51.9-percent to 48.1-percent in an “advisory” referendum on whether to remain in or leave the EU.

That result indicates a divided country, and closer inspection reveals even more serious divisions. The United Kingdom is a union of different entities; while England (53.4%) and Wales (52.5%) voted to leave, Scotland (62%) and Northern Ireland (55.8%) to remain.

The vote created important political, social, and economic challenges – and also legal ones. The referendum, being advisory, did not formally initiate a legal process to leave the European Union. It quickly became apparent that this technical issue was not straightforward. From the EU’s perspective, the Lisbon Treaty provides a mechanism to leave in Article 50: “Any Member State may decide to withdraw from the Union in accordance with its own constitutional requirements.” (emphasis added).

There is something ironic about this language, at least when it is applied to the UK, with its unusual constitutional framework – variously described as “unwritten,” “uncodified,” or “political.” In an uncodified political constitution, just what are the UK’s “own constitutional requirements”?  

The British government thought it could use the “royal prerogative” – that “residue” of monarchical power which the government exercises in the Queen’s name – to start the process of leaving the EU. The government generally conducts foreign affairs through the prerogative, so this makes some sense. But, there are complications; the European Union is not a typical international organization for starters. There is another complication: the UK’s constitution is based on parliamentary sovereignty; specifically, an Act of Parliament is the highest source of law in the UK legal system and so can abolish or suspend prerogative powers.

The Brexit litigation

In the Brexit litigation, applicants (plaintiffs) in England argued that an Act of Parliament – the European Communities Act 1972 (ECA 1972) – precluded the use of royal prerogative to trigger Article 50. Applicants in Northern Ireland argued the same for the Northern Ireland Act 1998 and the Belfast/Good Friday Agreement. In addition, Northern Irish parties and Law Officers from Scotland and Wales further claimed that the UK Parliament needed to consult with and obtain the consent of the devolved assemblies to any such legislation.

The Northern Ireland High Court rejected the arguments in the cases before it. However shortly afterwards the High Court in England and Wales ruled in favor of the applicants. This immediately became front page news. Indeed the judgment provoked hysterical reaction among some media that judges were “enemies of the people;” some politicians exacerbated tensions by calling for a march on the Supreme Court. Government ministers who had a legal obligation to protect judicial independence were lackluster in their response.

On Tuesday, the Supreme Court handed down its decision in these cases. In an 8-3 majority judgment, the Supreme Court decided that Article 50 cannot be triggered by the prerogative—an Act of Parliament is required. The majority decided this on the basis of the ECA 1972 and did not find it necessary to decide this point on the Northern Ireland Act and Agreement. The Court unanimously rejected the argument that there was a legal obligation to consult the devolved assemblies.

The Court was clearly aware of the wider political context in which its decision was being handed down. The Supreme Court consistently affirmed that the judgment was not about whether Brexit should take place but about the proper legal process. In a highly unusual move, the remarkable majority opinion is authored by all 8 judges; this clearly indicates the determination to present a robust and unified decision and not to give naysayers opportunities to claim the judgment was ambiguous or uncertain.


The road ahead for Brexit is now somewhat clearer. An Act of Parliament (not a resolution or some other mechanism) is required to trigger Article 50. This sends the issue of how to proceed back to Parliament. Absent a referendum it is likely that Brexit would have never secured a Parliamentary majority. Post-referendum it is unlikely that Parliament will vote against Brexit. Many MPs will feel that they should respect the vote in June 2016; or they may come from a constituency where the majority voted to leave.

However, there is considerable opportunity for parties that disagree with the result or disagree with how Brexit is being handled to influence the debate. Already some parties in Parliament are saying they will demand a second referendum, or will table 50 amendments to any Brexit Bill. There is now likely to be much greater scrutiny of the Government’s Brexit negotiating position.

It is also worth remembering that as well as the democratically elected House of Commons, the UK Parliament includes an unelected second Chamber, the House of Lords. The Lords includes an expert-based Select Committee on the EU and the government might expect probing questions in that chamber. The House of Lords is unlikely to defy the Commons on this issue (due to the Lords’ lack of democratic legitimacy) but if they do, this could lead to a clash between the two chambers. If so the Parliament Act procedure means that ultimately the Commons wins, but the Lords can delay the legislation for a year, which would take us closer to the next scheduled UK general election in 2020.

Nor have all the legal issues been settled. The parties in the UK Supreme Court all accepted that Article 50 is irreversible: once triggered it leads to the Member State leaving the EU after two years (unless the other states agree otherwise). But whether that is the correct understanding is a matter of EU Law, and ultimately for the Court of Justice of the EU, not the UK Supreme Court, to decide. There is now an effort to bring this issue before a Dublin court so as to litigate this issue at the Court of Justice. If the Court of Justice examines this question, it might conclude that an Article 50 notice can be withdrawn; this again would change the dynamics of the Brexit negotiating process.

More uncertainties lurk. The devolved nations (Scotland and Northern Ireland), or at least many in them, have expressed concern at the Supreme Court’s rejection of a legal obligation to consult them on an Act of Parliament to give effect to Brexit. There is a convention (a political custom which is treated as politically binding but not legally enforceable) requiring consultation but no legal requirement. This may increase the possibility of a second Scottish independence referendum. Similarly, the judgment offers little discussion on the unique features of Northern Ireland’s constitutional settlement.  

The strength of the majority judgment is that it rests on a traditional, axiomatic principle of the UK constitution – an Act of Parliament is the highest source of law. From the viewpoint of the devolved nations of Northern Ireland and Scotland, that principle is a source of political instability. For example, nothing provides legal protection for the devolution settlements or for Northern Ireland’s distinctive post-conflict constitution from an Act of Parliament that may be adopted notwithstanding objections from the Northern Ireland Assembly or the Scottish Parliament. The Scottish First Minister puts it bluntly: the implication is that constitutional guarantees for Scotland (or Northern Ireland for that matter) in Acts of Parliament are “not worth the paper they are written on.”

These concerns suggest the Brexit referendum result may yet have more serious unintended consequences for the UK’s political constitution.

Image: Ben Pruchnie/Getty