The Supreme Court ruled today by a majority of 8-3 that an Act of Parliament is needed for the UK Government to trigger Article 50 and formally begin the process of leaving the EU. Giving the leading judgment the President of the Court, Lord Neuberger, emphasised that the judgment was not about whether the UK should leave the EU but what was necessary to respect the UK’s constitutional arrangements.

The formal case title and citation is R (on the application of Miller and Dos Santos) v Secretary of State for Exiting the European Union and associated references [2017] UKSC 5. A press summary is available as well as the full judgment.

The claimants argued that an Act of Parliament was necessary because withdrawal from the EU would remove fundamental rights and the Government could not unilaterally do so relying only on the Crown’s perogative power. This principle goes back at least as far as the seventeenth century Case of Proclamations (1610) 12 Co Rep 74 in which Sir Edward Coke CJ held

the King by his proclamation or other ways cannot change any part of the common law, or statute law, or the customs of the realm.

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The Government has argued that Ministers could simply take the decision to leave the EU without any need for formal Parliamentary approval because the UK’s membership of the EU was the result of an international treaty and the perogative power could and is used to sign and withdraw from international treaties.

The Government’s arguments were rejected by the majority of judges. At paragraph 91 of the judgment Lord Neuberger points out how implausible the position adopted by the Government was in constitutional terms:

The improbability of the Secretary of State’s case is reinforced by the point that, if, as he contends, prerogative powers could be invoked in relation to the EU Treaties despite the provisions of the 1972 Act, it would have been open to ministers to take such a course on or at any time after 2 January 1973 without authorisation by Parliament. It would also follow that cokeministers could have taken that course even if there had been no referendum or indeed, at least in theory, even if any referendum had resulted in a vote to remain. Those are implausible propositions.

The judges held at paragraph 81 that the Government case did not address the fundamental constitutional changes which would occur if Article 50 was triggered:

As we have said, withdrawal is fundamentally different from variations in the content of EU law arising from further EU Treaties or legislation. A complete withdrawal represents a change which is different not just in degree but in kind from the abrogation of particular rights, duties or rules derived from EU law. It will constitute as significant a constitutional change as that which occurred when EU law was first incorporated in domestic law by the 1972 Act. And, if Notice is given, this change will occur irrespective of whether Parliament repeals the 1972 Act. It would be inconsistent with long-standing and fundamental principle for such a far-reaching change to the UK constitutional arrangements to be brought about by ministerial decision or ministerial action alone. All the more so when the source in question was brought into existence by Parliament through primary legislation, which gave that source an overriding supremacy in the hierarchy of domestic law sources.

The key passages of the judgment come at paragraphs 121 and 122, where the judges rule that an Act of Parliament is needed, albeit not necessarily a very long one:

121. Where, as in this case, implementation of a referendum result requires a change in the law of the land, and statute has not provided for that change, the change in the law must be made in the only way in which the UK constitution permits, namely through Parliamentary legislation.

122. What form such legislation should take is entirely a matter for Parliament. But, in the light of a point made in oral argument, it is right to add that the fact that Parliament may decide to content itself with a very brief statute is nothing to the point. There is no equivalence between the constitutional importance of a statute, or any other document, and its length or complexity. A notice under article 50(2) could no doubt be very short indeed, but that would not undermine its momentous significance. The essential point is that, if, as we consider, what would otherwise be a prerogative act would result in a change in domestic law, the act can only lawfully be carried out with the sanction of primary legislation enacted by the Queen in Parliament.

The judges also ruled that the devolution settlements do not require that the devolved governments of Scotland, Wales or Northern Ireland must consent to the UK leaving the EU.

Following criticism of her failure to defend the judges who reached a similar conclusion previously in the High Court, Lord Chancellor Liz Truss released a statement defending the Supreme Court justices:

Our independent judiciary is the cornerstone of the rule of law and is vital to our constitution and our freedoms. The reputation of our judiciary is unrivalled the world over, and our Supreme Court justices are people of integrity and impartiality.

While we may not always agree with judgments, it is a fundamental part of any thriving democracy that legal process is followed. The government has been clear that it will respect the decision of the court.

The big question is what happens next. Leader of the Labour Party Jeremy Corbyn, a long standing opponent of the EU who voted against membership in 1975 and against the Maastrict and Lisbon Treaties, immediately announced that the Labour Party would not oppose any vote in Parliament to leave the EU. This makes it highly unlikely that, despite the slender Government majority of only 17, any Act will be voted down.

The Government quickly stated that Article 50 would be triggered by the end of March 2017 as originally planned.