The Supreme Court has spoken. The effect of the Prime Minister’s decision to prorogue Parliament was to frustrate Parliament from carrying out its constitutional functions as a legislature and holding the Government to account.

That made the Prime Minister’s decision unlawful and it is as if the prorogation, or suspension of Parliament, never happened. Parliament can now sit.

This is a remarkable judgment, all the more so when, against all predictions, the eleven justices were unanimous.  

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There will inevitably be political reaction to it. But it is also important to bear in mind what the Supreme Court has and has not done.

First, the court prudently side-stepped the question of whether the Prime Minister used the power to prorogue for an improper purpose.

All the court said – all it needed to say – was that the effect of the Prime Minister’s decision was to frustrate Parliament’s supervisory function whilst the country decides when and how to leave the European Union.

Second, through its President, Lady Hale, the court has been at pains to point out that this case is not about Brexit. That will be for Parliament when it returns to work, as it surely must.

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Third, the Supreme Court has decided this case on established public-law principles.

It has, rightly, put aside some of the rhetoric that has been deployed in this case. That was never going to work before the Supreme Court, which has long favoured calm, measured advocacy.

Instead, the Supreme Court applied those well-established principles, looked at the evidence before it and decided what the effect of the Prime Minister’s decision was on Parliament and on the Constitution.

Once it did that, the Supreme Court’s duty to find against the Government was clear.

It was once said that we pay the Supreme Court to decide the hard cases. The justices have done so: calmly, firmly and unanimously.

Paul Harvey is an advocate and barrister specialising in public law