London, Boris Johnson's decision to suspend Parliament for five weeks was unlawful, the Supreme Court has ruled.
Judges on Tuesday 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, said a BBC News report.
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.
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.
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.