Boris Johnson's suspension of Parliament has been ruled as unlawful because it was an "improper" attempt to "stymie Parliament", a court has ruled.
Judges at Scotland's highest civil court said the prorogation, which came into force on Monday, has "no effect" because it was pushed through unlawfully.
The UK Government plans to appeal against the latest ruling, which a spokesma said it was "disappointed" by, to the Supreme Court.
The legal bid, brought to the Edinburgh court by a group of around 70 parliamentarians, is separate to the case brought in the High Court by Gina Miller and John Major last week . Judges in London are set to give their reasons for dismissing this case later today.
Shadow Brexit secretary Keir Starmer said the court ruling was "huge", and vindicated Labour's efforts to stop Parliament being shut down.
— Luciana Berger (@lucianaberger)September 11, 2019
Speaking at the TUC Congress in Brighton, he said: "I need to get back to Parliament, to see if we can reopen the doors and hold Johnson to account.
"It was obvious to everyone that shutting down Parliament at this crucial time was the wrong thing to do.
"The Prime Minister was not telling the truth about why he was doing it. The idea of shutting down Parliament offended everyone across the country, and then they felt they were not being told the truth."
SNP MP Joanna Cherry tweeted: "All 3 judges in Scotland’s Highest court of appeal rule #Prorogation #unlawful! #Cherrycase succeeds."
She added: "Huge thanks to all our supporters & our fantastic legal team who have achieved the historic ruling."
— Joanna Cherry QC MP (@joannaccherry)September 11, 2019
Liberal Democrats MP Luciana Berger tweeted: "As one of the Petitioners to this case, this is such an important ruling - although how awful that it's had to come to this. "
SNP Westminster leader Ian Blackford tweeted: "This is great news, congratulations to you and all involved. This battle has further to run but my message to @BorisJohnson is you are playing fast and loose with the law. You have acted in an anti democratic manner and need to respond by recalling Parliament."
Jolyon Maugham QC, the anti-Brexit barrister who was second petitioner in the case, said the Supreme Court would hear the case next week.
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He 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."
"We need to get back representing the people who elected us in this time of political crisis."@theSNP's Tommy Sheppard and Joanna Cherry say their "next move is to demand the recall of parliament".
The PM's suspension has been ruled as unlawful: https://t.co/GOvEyOlrNU pic.twitter.com/zqWcuCS8Xf— Sky News Politics (@SkyNewsPolitics)September 11, 2019
He told Sky News said he expects the Supreme Court will agree with the appeal judge's position.
A Government spokesman the suspension was "necessary" to allow a new legislative agenda to be proposed.
He 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."
Judge Lord Doherty originally dismissed a challenge against the suspension - which went ahead in the early hours of Tuesday - at the Court of Session last Wednesday, saying it is for politicians and not the courts to decide.
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But three judges of the Inner House, the supreme civil court in Scotland, disagreed with Lord Doherty's ruling.
A summary of the court opinion, published by the Scottish Courts and Tribunals Service, states: "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 9 and 12 September until 14 October was unlawful because it had the purpose of stymying Parliament."
It continues: "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."
It went on: "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."