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 said he “profoundly disagreed” with the ruling but would “respect” it.
A raft of MPs have now called for the prime minister to resign and some say they will attempt to force him out if he does not go of his accord.
Mr Johnson insisted he wanted to outline his government’s policies in a Queen’s Speech on 14 October, and to do that, Parliament must be prorogued and a new session started.
But critics said he was trying to stop MPs scrutinising his Brexit plans and the suspension was far longer than necessary.
During a speech in New York, the PM said he “refused to be deterred” from 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 carrying out its duties “without reasonable justification”.
A No 10 source said the Supreme Court had “made a serious mistake in extending its reach to these political matters”, and had “made it clear that its reasons [were] connected to the Parliamentary disputes over, and timetable for” Brexit.
But Lady Hale emphasised in the ruling that the case was “not about when and on what terms” the UK left the EU – it was about the decision to suspend Parliament.
Delivering the justices’ conclusions, she 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.
Speaker of the Commons John Bercow said MPs needed to return “in light of the explicit judgement”, 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: “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.
Labour leader Jeremy Corbyn was due to close the Labour Party conference in Brighton with a speech on Wednesday, but brought it forward to Tuesday afternoon so he could return to Westminster.
He told cheering delegates: “Tomorrow Parliament will return. The government will be held to account for what it has done. Boris Johnson has been found to have misled the country. This unelected prime minister should now resign.”
Lawyers for the government had argued the decision to prorogue was one for Parliament, not the courts.
But the justices disagreed, unanimously deciding it was “justiciable”, and there was “no doubt that the courts have jurisdiction to decide upon the existence and limits of a prerogative power”.
The court also criticised the length of the suspension, with Lady Hale saying it was “impossible for us to conclude, on the evidence which has been put before us, that there was any reason – let alone a good reason – to advise Her Majesty to prorogue Parliament for five weeks”.
A spokesperson from the Attorney General’s office said the government had acted in “good faith and in the belief that its approach was both lawful and constitutional”.
“These are complex matters on which senior and distinguished lawyers have disagreed,” a statement said.
“The Divisional Court led by the Lord Chief Justice agreed unanimously with the Government’s legal position, as did the Outer House in Scotland.
“We are disappointed that in the end the Supreme Court took a different view. We respect the judgment of the Supreme Court.”