Boris Johnson is facing furious demands for the immediate recall of MPs to Westminster after the suspension of Parliament was ruled unlawful by Scotland’s highest civil court.
In a dramatic judgment, the Court of Session in Edinburgh found ministers had stopped MPs from sitting for the “improper purpose of stymying Parliament”.
It said advice given by ministers to the Queen which led to the five-week prorogation was therefore “unlawful and is thus null and of no effect”.
The Government immediately announced it was lodging an appeal against the ruling with the Supreme Court, with a hearing set for Tuesday.
However opposition MPs said the prorogation should be set aside without delay so ministers could be held to account for their Brexit plans in the Commons.
Shadow Brexit secretary Sir Keir said: “What Boris Johnson should do is to urgently recall Parliament.
“We should be back there this afternoon, or tomorrow, so we can debate this judgment, and we can decide what to do next.
“He should do the right thing now, which is to reopen Parliament, let us back to do our job and to decide what to do next.
“That is within his power, and we must take the decisions when we are recalled and back actually doing the job we are sent to Parliament to do.”
Former attorney general Dominic Grieve warned that if ministers had misled the Queen over the reasons for prorogation, Mr Johnson’s position would be untenable and he would have to resign.
“It is absolutely central to our constitution that the relationship between the Prime Minister and the Queen is one of the utmost confidentiality and the utmost good faith. Central,” he told the BBC.
“So, if it were to be the case that the Government had misled the Queen about the reasons for suspending Parliament and the motives for it, that would be a very serious matter indeed.
“Indeed in my view, it would then be the moment for Mr Johnson to resign and very swiftly.”
Downing Street insisted the five-week prorogation was so the new Government could set out its legislative programme in a Queen’s Speech on October 14.
The Prime Minister’s official spokesman rejected claims that the true purpose was to prevent MPs thwarting his pledge to take Britain out of the EU by October 31, with or without a deal with Brussels.
“We have set out the reasons in public why we have prorogued. That is to allow us to bring forward a new legislative programme,” the spokesman said.
The spokesman added that the Government would abide by the ruling of the Supreme Court, which is also considering an appeal against a ruling by the High Court in London which found that the suspension was lawful.
In the meantime, officials said Parliament would remain prorogued.
The unanimous ruling by three judges sitting at the Court of Session followed an appeal by a group of around 70 parliamentarians who had appealed against an earlier ruling that the prorogation was lawful.
Judge Lord Carloway told the court: “We are of the opinion that the advice given by the Government to her majesty the Queen to prorogue Parliament was unlawful and that the prorogation itself was unlawful.”
In contrast, the High Court judges – issuing the reasons for their ruling in the case brought by businesswoman Gina Miller – said on Wednesday that it was “not a matter for the courts”.
The judges said the decision to prorogue and the advice given to the Queen were “inherently political in nature” and that there were “no legal standards against which to judge their legitimacy”.
SNP MP Joanna Cherry, who led the case in the Scottish courts, said Mr Johnson must now bring MPs back to Westminster.
“The court agreed it is unlawful to suspend the UK Parliament for the specific purpose of preventing Parliament from scrutinising the Brexit process and holding this shambolic Tory Government’s extreme Brexit plans to account,” she said.
“We have uncovered more and more evidence that this was a plot by Boris Johnson and his cronies to prevent us from stopping them taking Scotland and the UK off a Brexit cliff edge by forcing through a damaging no deal against the will of Parliament.
“Boris Johnson cannot be allowed to break the law with impunity.”
Liberal Democrat Brexit spokesman Tom Brake warned ministers could not ignore the implications of the ruling.
“The prorogation of Parliament was never more than a power grab,” he said.
“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.”