Boris Johnson’s decision to suspend Parliament has been ruled “improper” and “unlawful” by three senior Scottish judges, who concluded it had been done with “the purpose of stymying Parliament”.
The judges at the Court of Session in Edinburgh said the prorogation was therefore “null and of no effect”.
The UK Government said it was “disappointed” by the decision and would be appealing to the UK’s Supreme Court.
A number of MPs are now calling for Parliament to be recalled in the wake of the ruling.
Meanwhile, a separate court hearing at the High Court in London rejected a case brought by businesswoman Gina Miller, ruling the decision to prorogue Parliament was “purely political” and therefore not capable of challenge in the courts.
This judgment will also be challenged in the UK’s Supreme Court.
The Court of Session case, brought by a group of more than 70 parliamentarians, appealed against an earlier ruling by a judge that the Prime Minister’s prorogation was lawful.
Judge Lord Doherty dismissed the challenge at the Court of Session last Wednesday, saying it was for politicians and not the courts to decide.
But three judges at the Inner House, the supreme civil court in Scotland, disagreed with Lord Doherty’s ruling on Wednesday.
A summary of the court opinion, published by the Scottish Courts and Tribunals Service, stated: “The Inner House of the Court of Session has ruled that the Prime Minister’s advice to HM the Queen that the United Kingdom Parliament should be prorogued from a day between September 9 and 12 until October 14 was unlawful because it had the purpose of stymying Parliament.
“All three First Division judges have decided that the PM’s advice to the HM the Queen is justiciable, that it was motivated by the improper purpose of stymying Parliament and that it, and what has followed from it, is unlawful.
“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.”
At the hearing, 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.”
He referred the matter to the UK Supreme Court for resolution.
The ruling comes a day after the prorogation took place in the early hours of Tuesday, with Parliament now suspended for five weeks.
The group of parliamentarians who petitioned the Court of Session said their understanding is Parliament can now reassemble if it so wishes, with SNP MP Joanna Cherry among those calling for it to be recalled.
She said:“The majority of MPs want to be in the House scrutinising and debating Brexit.
“Mr Johnson doesn’t have a majority, the Speaker is very concerned about the circumstances of the prorogation, his concerns have been vindicated by this ruling and therefore Parliament should return and we should be able to get on with our jobs representing our constituents.”
The campaigners included Liberal Democrat leader Jo Swinson, Labour MP Ian Murray and anti-Brexit barrister Jo Maugham, of the Good Law Project.
Mr Maugham tweeted: “We have won. Appeal begins in the Supreme Court on Tuesday.
“We believe that the effect of the decision is that Parliament is no longer prorogued.
“I have never been able to contemplate the possibility that the law could be that our sovereign Parliament might be treated as an inconvenience by the Prime Minister.
“I am pleased that Scotland’s highest court agrees. But ultimately, as has always been the case, it’s the final arbiter’s decision that matters.
“We will convene again in the Supreme Court next week.”
A UK Government spokesman said: “We are disappointed by today’s decision, and will appeal to the UK Supreme Court.
“The UK Government needs to bring forward a strong domestic legislative agenda.
“Proroguing Parliament is the legal and necessary way of delivering this.”
Meanwhile Justice Secretary Robert Buckland, commenting after reports a Downing Street source had suggested the Scottish judges were “politically biased”, said: “Our judges are renowned around the world for their excellence and impartiality and I have total confidence in their independence in every case.”