Boris Johnson is facing calls to resign as prime minister after the Supreme Court ruled he suspended Parliament unlawfully.
There were chants of “Johnson out!” in the hall at the Labour Party conference in Brighton after the ruling.
Eleven judges ruled unanimously on Tuesday that Mr Johnson’s five-week prorogation of Parliament was illegal.
The ruling puts enormous pressure on Mr Johnson’s position, with calls made for his resignation almost immediately.
Speaking at his party’s conference, Labour leader Jeremy Corbyn said Mr Johnson had “acted wrongly in shutting down Parliament”.
He said: “I invite Boris Johnson, in the historic words, to consider his position and become the shortest serving prime minister there’s ever been”.
Labour’s deputy leader, Tom Watson, tweeted: "Boris Johnson should resign."
SNP MP Joanna Cherry, who led the original case against Parliament’s suspension in the Scottish courts, said Mr Johnson's position was "untenable" and called on him to resign.
Chants of “Johnson Out!” at the Labour conference. pic.twitter.com/5bRoYTzTKS— Peter Saull (@petesaull) September 24, 2019
She said: "This is an absolutely momentous decision. There is nothing to stop us - Members of Parliament - resuming immediately the important job of scrutinising this minority Tory government as we hurtle towards Brexit.
“This is a huge victory for the rule of law and for democracy. As regards Mr Boris Johnson, the highest court in the United Kingdom has unanimously found that his advice given to Her Majesty the Queen was unlawful.
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"His position is untenable and he should have the guts for once to do the decent thing and resign."
Boris Johnson “must resign”
Ian Blackford, leader of the SNP in Westminster, also called on Mr Johnson to resign as prime minister.
"We must be back in Parliament immediately," he said. "We want to get back to work. On the back of this, Boris Johnson must resign."
Senior Labour MP Angela Eagle said: “He should resign: he has been found lying and cynical - he's lied to the Queen. He's used cynical reasons to cover up what is his attempt to gag Parliament.
"He has been found out by the highest court of the land by a unanimous verdict. He must now resign."
Dominic Cummings “must go”
Brexit Party leader Nigel Farage called for the prime minister's chief adviser Dominic Cummings to be removed from his role following the Supreme Court's decision.
"The calling of a Queen's Speech and prorogation is the worst political decision ever," he tweeted. "Dominic Cummings must go."
Before the Supreme Court decision, Mr Johnson did not rule out resigning if it decided he had acted unlawfully.
Asked if he would resign if the government lost the case, he said: "I will wait and see what the justices decide, the Supreme Court decides, because as I've said before I believe that the reasons for… wanting a Queen's speech were very good indeed.”