It’s not entirely clear precisely how Mischons propose to stymie a British PM pulling the Article 50 trigger. All they say is:
“Legal steps have been taken to ensure the UK Government will not trigger the procedure for withdrawal from the EU without an Act of Parliament. The case is being brought by leading law firm, Mishcon de Reya, on behalf of a group of clients”.
The first issue therefore is whether a PM is required under the (unwritten) UK Constitution to put the decision to trigger Article 50 to Parliament for a vote.
The proposition reminds me of Ex p Molyneaux  1 WLR 331.
James Molyneaux was a Northern Ireland politician who was unhappy with the 1985 Anglo-Irish Agreement. He was of the view that the Agreement should have been approved by a Parliamentary vote, rather than by Cabinet. Mr Molyneaux and others attempted to judicially review the UK Government’s decision to sign the Agreement without obtaining Parliamentary approval.
The Court in Molyneaux recognised the prerogative power to negotiate treaties, and held that the exercise of that power was, “non-justiciable” – which is a college word for saying it’s none of the Court’s business.
What’s a prerogative power, you ask? They are “residual powers” from the “Divine Right”, exercised in the name of the Queen by the PM and her/his Ministers which don’t really lend themselves to statute. For example, the prerogative powers include the power to grant peerages, defence of the realm, declarations of war and foreign affairs.
In plain English, that means that because it wouldn’t really work for the PM to have to get Parliamentary approval for they way in which Ministers defend the nation etc, the residual right remains. Further, when it comes to matters of foreign affairs or defence of the realm, the Court is highly reluctant to get involved.
Getting back to Article 50, the Lisbon Treaty states that it is up to the Member State itself to decide whether the triggering method it adopts is compliant with its own constitution.
The David Lammy Solution
I agree. It should go to Parliament for a vote.
But in the absence of a written constitution and in light of the Molyneaux case, it is difficult for me – at this stage – to see how we can stop a future PM pulling the Article 50 trigger prior to a majority vote in Parliament.
While the Constitutional Reform and Governance Act 2010 sets out requirements for the ratification of treaties, that isn’t the same as the PM triggering a Brexit.
I wish I could see a rabbit and a hat – but unless the next PM continues to kick Article 50 into the long grass, I don’t see how else, “we the people” can stop it. I further can’t see the judiciary making Brexit the exception to the general non-justiciability re foreign affairs rule.
NB 4 July 2016
Thanks to Law lecturer Stephanie Mindel for alerting me to R v SS for Foreign & Commonwealth Affairs ex p Lord Rees Mogg where Lord Rees Mogg judicially reviewed the Government’s decision to ratify the Maastricht Treaty. We note that Pannick QC was counsel for Rees Mogg.
As Mr Kentridge succinctly put it, Title V does not entail an abandonment or transfer of prerogative powers; but an exercise of those powers. We agree. So far as we know, nobody has ever suggested that the Charter of the United Nations, for example, or of the North Atlantic Treaty Organisation, involves a transfer of prerogative powers. Title V should be read in the same light. In the last resort, as was pointed out in argument, though not pursued, it would presumably be open to the Government to denounce the Treaty, or at least to fail to comply with its international obligations under Title V.