Boris Johnson’s decision to suspend Parliament for five weeks was unlawful, the Supreme Court has ruled.
Judges said it was wrong to stop MPs carrying out duties in the run-up to the Brexit deadline on 31 October.
The PM, who has faced calls to resign, said he “profoundly disagreed” with the ruling but would “respect” it.
The Labour conference finished early following the ruling and MPs are returning to Westminster ready for Parliament to reconvene on Wednesday.
A senior government official said the prime minister spoke to the Queen after the Supreme Court ruling, but would not reveal the details of the conversation.
It comes after the court ruled it was impossible to conclude there had been any reason “let alone a good reason – to advise Her Majesty to prorogue Parliament for five weeks”.
Mr Johnson, who returns to London from New York on Wednesday, also chaired a 30-minute phone call with his cabinet.
A source told the BBC that the Leader of the Commons Jacob Rees-Mogg said to other cabinet ministers on the call that the action by the court had amounted to a “constitutional coup”.
The prime minister 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.
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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 Supreme Court president 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.
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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.
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.”