Jeremy Corbyn is leading calls for Boris Johnson to resign after the Supreme Court ruled the PM’s decision to suspend Parliament was unlawful.
The Labour leader said Mr Johnson should “consider his position” in a short statement to the party’s conference in Brighton.
The SNP and some Labour MPs have said the PM could be ousted in a no confidence vote, if he refuses to go.
MPs will return to work on Wednesday at 11:30 BST after the court’s ruling.
Mr Johnson suspended – or prorogued – Parliament for five weeks earlier this month, but the court said it was wrong to stop MPs carrying out their duties in the run-up to Brexit on 31 October.
The PM has promised the UK will leave on that date, with or without a deal with the EU, but Parliament has made clear it does not want to leave on a no-deal basis.
Supreme Court president Lady Hale said “the effect [of prorogation] on the fundamentals of democracy was extreme” and the government had provided no justification for it.
In a statement outside Parliament, Commons Speaker John Bercow said there would be no Prime Minister’s Questions on Wednesday, but there would be scope for urgent questions, ministerial statements and emergency debate applications.
Opposition MPs say they plan to use the session to hold Mr Johnson to account for his decision to suspend Parliament – and potentially begin moves to oust him has prime minister.
Liberal Democrat leader Jo Swinson said Mr Johnson’s actions had shown he was “not fit to be prime minister” and he should resign.
The SNP’s leader at Westminster, Ian Blackford, said: “If he doesn’t go, Parliament will have to remove Boris Johnson.”
Labour MP Stephen Kinnock said opposition parties were likely to table a vote of no confidence in the prime minister once they could be sure a no-deal Brexit had been ruled out.
He said Mr Johnson was unlikely to stand down of his own free will and would “squat in Number 10,” adding that MPs would have to “take matters intro our own hands in Parliament”.
Former Justice Secretary David Gauke was among the MPs expelled from the Conservative Party by Mr Johnson for voting against his Brexit plans, but he said he would not be calling for the PM to resign.
“I think the prime minister can survive,” he told BBC News, but only if he apologised for “what was done earlier this month in terms of proroguing Parliament” and changes his Brexit strategy and sacks chief adviser Dominic Cummings.
What could happen when MPs return?
Boris Johnson resigns – Highly unlikely given his defiant reaction to the Supreme Court verdict
No-confidence vote – If a majority of MPs back it and no alternative prime minister who can command a majority emerges within 14 days then there will be a general election
Censure motion – A non-binding slap on the wrist for the prime minister allowing MPs to register their disapproval of his actions without triggering his removal or an election
Impeachment – Being talked about by some Labour MPs but highly unlikely. No British prime minister has ever been impeached – only ministers – and the last attempted impeachment of a minister was in 1806
Jeremy Corbyn had been due to deliver his keynote speech at Labour’s conference on Wednesday but will now address delegates later on Tuesday.
Labour’s deputy leader Tom Watson, who had been due to speak on Tuesday, said: “This is a momentous day.
“It’s right that Jeremy closes the conference this afternoon. I’ll be in London tomorrow (Wednesday) to hold our law-breaking prime minister to account. I’ll save the speech I was going to make until next year.”
Earlier, Mr Corbyn told activists Mr Johnson would become “the shortest-serving prime minister there has ever been” if he resigned.
He added: “So, obey the law, take no-deal [Brexit] off the table and have an election to elect a government that respects democracy, that respects the rule of law and brings power back to the people – not usurps it in the way that Boris Johnson has done.”
At the end of Mr Corbyn’s short statement, Labour delegates chanted “Johnson out”.
There were emotional scenes outside the Supreme Court in London as opposition MPs and campaigners hailed the court’s unanimous verdict.
The SNP’s Joanna Cherry, who pursued the case through the Scottish courts, said it was an “absolutely momentous decision” that made Mr Johnson’s position as prime minister was “untenable”.
Green Party MP Caroline Lucas said: “Johnson must go and we must get back to work.”
Businesswoman and campaigner Gina Miller, who challenged Mr Johnson’s prorogation in the High Court, said the prime minister was not above the law.
She added: “MPs must get back and be brave and bold in holding this unscrupulous government to account.”
Conservative former Prime Minister Sir John Major, who took part in the Supreme Court case, said; “No prime minister must ever treat the monarch or Parliament in this way again.”
Brexit Party leader Nigel Farage described the prime minister’s decision to suspend Parliament as “the worst political decision ever” following the Supreme Court defeat.
He called for Mr Johnson’s chief adviser, Dominic Cummings, to quit.