Opposition MPs are demanding Parliament be recalled after a court ruling deemed its closure unlawful.
Parliament was suspended, or prorogued, in the early hours of Tuesday – something Boris Johnson said was normal practice for a new government.
But critics claimed his intention was to avoid scrutiny in the run-up to the Brexit deadline on 31 October.
Shadow Brexit secretary Sir Keir Starmer said Parliament should be recalled as early as this afternoon.
He told the BBC: “Most people didn’t believe Boris Johnson, but for the courts to find he has unlawfully shut down Parliament and that his motive wasn’t the one he said it was? That’s very powerful.
“I call on him to recall Parliament. Let’s get it back open, and sitting this afternoon and tomorrow, so we can debate what happens next and we can debate this judgement.”
The power to suspend Parliament rests with the Queen, but she is bound to act on the advice of her prime minister.
Former Tory MP Dominic Grieve – who had the Conservative whip removed after voting to block a no-deal Brexit – said if it was the case the government had mislead the Queen, Mr Johnson should resign.
But government sources told the BBC they rejected the demand to recall Parliament, arguing there remained conflicting legal views over prorogation.
Some MPs are returning to Parliament already, with Labour’s Kevin Brennan saying it was time for Prime Minister’s Questions.
Angry MPs protested in the Commons as Parliament was shut down for five weeks on Tuesday.
During the suspension, parties are due to hold their annual conferences but no debates, votes or official committee scrutiny sessions will take place.
In a summary of their findings, the Court of Session judges said they were unanimous in their belief that Mr Johnson was motivated in his decision to prorogue by the “improper purpose of stymying Parliament”.
They added: “The Court will accordingly make an Order declaring that the prime minister’s advice to HM the Queen and the prorogation which followed thereon was unlawful and is thus null and of no effect.”
The government said it would appeal the decision in the UK’s Supreme Court on Tuesday.
Sources added that those trying to recall Parliament were seeking to pre-empt the higher court’s decision.
But the SNP’s Joanna Cherry, who was one of the lawyers involved in the case in Scotland, said Parliament should be recalled “for the time being”.
She told the BBC that, even if the Supreme Court were to issue its decision immediately, “we will have lost about 10 days of Parliamentary time… so we in the SNP are calling for Parliament to be resumed”.
“We think Parliament should go back and get on with the job of scrutinising this government, looking at what they’re up to.”
Another body blow for Johnson
Whichever way you slice this, it’s a body blow for Boris Johnson.
Just look at the language the court uses – accusing him of trying to stifle Parliament and mislead the Queen.
They seem to be questioning his motivation – suggesting this is all about getting round Parliamentary scrutiny.
The big question is whether he will now have to recall Parliament.
It seems to me quite likely that he will, possibly even before the Supreme Court ruling on Tuesday. Perhaps today or tomorrow.
And then he would most likely have to put down a motion to ask MPs whether they want to go into recess for the conferences and there’s a very high chance they would say no.
Mr Grieve – was is a former Attorney General – said the ruling was a “serious indictment on the government”.
He told BBC News the judges made it “quite clear” that the explanation for suspending Parliament was “simply inaccurate and untrue”.
Mr Grieve added: “We have been making the same point in the House of Commons.
“It wasn’t to reset the government, to suspend Parliament for a short time and have a Queen’s Speech, it was to prevent us from holding the government to account.
“I would hope the government would now realise the extent of the crisis it’s created and recall Parliament immediately.”
The Liberal Democrats’ Brexit spokesman, Tom Brake, said the ruling was “highly embarrassing” for the prime minister, and showed prorogation was “never more than a power grab”.
He added: “It was an authoritarian move by Boris Johnson designed to overrule and silence the people and their representatives and to force a disastrous no-deal Brexit on our country.”