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.
The government said Parliament would remain prorogued pending its appeal to the Supreme Court next week.
Labour, the SNP and the Lib Dems are all demanding that MPs be recalled as early as Wednesday afternoon. They are not due to sit again until 14 October when the government had planned to hold a Queen’s Speech, setting out its policy agenda.
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 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 – called for Parliament’s immediate recall and said if it was the case the government had misled the Queen, Mr Johnson should resign.
Shadow Brexit secretary Sir Keir Starmer 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.”
SNP’s leader in Westminster Ian Blackford, who has written to the prime minister to insist Parliament is recalled, said: “Every day Parliament remains suspended, Boris Johnson and the UK government are shutting down democracy.”
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.
Following the court ruling, some MPs – including Labour’s Kevin Brennan – said they were returning to Parliament.
Labour’s Luke Pollard tweeted: “Quietly and peacefully I have gone back to sit in my usual spot in the House of Commons. No shouting or scuffles – just a quiet statement about our democracy.”
Government sources said those trying to recall Parliament were seeking to pre-empt the appeal on Tuesday.
No 10 also denied reports that sources had been critical of the Scottish judges.
Justice Secretary Robert Buckland later tweeted he had “total confidence” in the court’s independence.
Leader of the House Jacob Rees-Mogg said: “We are in very complicated legal territory but we should know the outcome next week.”
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.”
The BBC’s political editor, Laura Kuenssberg, said the law in Scotland was different to that in England and the ruling would put “massive pressure” on the Supreme Court over who to side with.
A bombshell moment?
The response of Team Johnson this morning has been to say “calm down, we are not recalling Parliament now, just wait and see what the Supreme Court decides.”
But any way you look at it, this is bad for the prime minister and has the potential to become a full-blown bombshell moment for him.
Bad because he has been judged to have behaved unlawfully and, more than that, the judges clearly believe he misled Parliament.
If the Supreme Court upholds their view, it seems he will have no option but to recall Parliament which would be a political humiliation for him.
On top of that, MPs will almost certainly vote to scrap the party conference recess and sit through September right up to the 31 October Brexit deadline.
The bottom line, to put it mildly, is that an awful lot is hanging on what the judges decide on Tuesday.
Mr Grieve – a former Attorney General – called the ruling 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”.
Guto Bebb, another Conservative rebel stripped of the whip last week, said No 10 must now release all internal communications relating to the decision to suspend Parliament as demanded by MPs in a vote on Monday.
“We now have a question as to the integrity and honesty of the Downing Street operation,” he told Radio 4’s World at One.
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”.
But Brexit Party leader Nigel Farage said the decision “smelt” of judicial interference.
“How can a Queen’s Speech be unlawful?” he said. “The establishment will stop at nothing to frustrate the will of the people.”