Boris Johnson’s decision to suspend Parliament was unlawful, the Supreme Court has ruled.
Mr. Johnson suspended – or prorogued – Parliament for five weeks earlier this month, but judges said it was wrong to stop MPs carrying out duties in the run-up to Brexit on 31 October.
Supreme Court president Lady Hale said, “the effect on the fundamentals of democracy was extreme.”
The PM says he “strongly disagrees” with the ruling but will “respect” it.
A raft of MPs have now called for the prime minister to resign and some have said they would attempt to force him out if he did not go of his accord.
Mr. Johnson argued he wanted to carry out the prorogation so he could outline his government’s new policies in a Queen’s Speech.
But critics said he was trying to stop MPs from scrutinizing his Brexit plans and the suspension was far longer than necessary.
At a speech in New York, the PM said he “refused to be deterred” on getting on with “an exciting and dynamic domestic agenda”, and to do that he would need a Queen’s Speech.
The court ruling does not prevent him from proroguing again in order to hold one, as long as it does not stop Parliament from carrying out its duties “without reasonable justification”.
Delivering its conclusions, the Supreme Court’s president, Lady Hale, said: “The decision to advise Her Majesty to prorogue Parliament was unlawful because it had the effect of frustrating or preventing the ability of Parliament to carry out its constitutional functions without reasonable justification.”
Lady Hale said the unanimous decision of the 11 justices meant Parliament had effectively not been prorogued – the decision was null and of no effect.
She added that it was important to emphasize the case was “not about when and on what terms” the UK left the EU, but about the decision to suspend Parliament.
Speaker of the Commons John Bercow said MPs needed to return “in light of the explicit judgment”, and he had “instructed the House of Commons authorities to prepare… for the resumption of business” from 11:30 BST on Wednesday.
He said prime minister’s questions would not go ahead, but there would be “full scope” for urgent questions, ministerial statements, and applications for emergency debates.
Short of the inscrutable Lady Hale, with the giant diamond spider on her lapel, declaring Boris Johnson to be Pinocchio, this judgement is just about as bad for the government as it gets.
Mr Johnson is, as is abundantly clear, prepared to run a general election campaign that pits Parliament against the people. And so what, according to that view of the world, if that includes the judges as part of the establishment standing in his way?
But there is a difference between being ruthless and reckless.
And the scope and strength of this judgement cannot just be dismissed as some pesky judges sticking their noses in.
Reacting to the ruling, Mr Johnson said it was an “unusual judgement”, adding: “I don’t think this was the right decision. The prerogative of prorogation has been used for centuries without this kind of challenge.
“There are a lot of people who basically want to stop this country from coming out of the EU and we have a Parliament that is unable to be prorogued and doesn’t want to have an election. I think it is time we took things forward.”
The PM said getting a deal was “not made much easier with these sort of things in Parliament or the courts”, but insisted the UK would still leave on 31 October.
Pushed on whether he would attempt to suspend Parliament again, he said there was “a good case for getting on with a Queen’s Speech anyway”, and the Supreme Court had not “remotely excluded” the possibility.
Labour leader Jeremy Corbyn said the ruling showed Mr Johnson’s “contempt for democracy”, adding: “I invite Boris Johnson, in the historic words, to consider his position.”
Mr Corbyn was due to close the Labour Party conference in Brighton with a speech on Wednesday but has brought it forward to Tuesday afternoon so he can return to Parliament.