Scotland's highest court of appeal has ruled that British Prime Minister Boris Johnson's decision to suspend parliament for five weeks is unlawful.
A group of around 70 parliamentarians had appealed against a ruling by a judge at the court that Mr Johnson's prorogation of parliament was lawful.
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.
But three judges of the Inner House, the supreme civil court in Scotland, disagreed with Lord Doherty's ruling.
Mr Johnson's advice to Queen Elizabeth that parliament be prorogued until 14 October "was unlawful because it had the purpose of stymying parliament", a summary of the judgment said.
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."
The campaigners included SNP MP Joanna Cherry, Liberal Democrat leader Jo Swinson, Labour MP Ian Murray and anti-Brexit barrister Jo Maugham, of the Good Law Project.
"Huge thanks to all our supporters & our fantastic legal team who have achieved the historic ruling that #prorogation is #unlawful," Ms Cherry said on Twitter.
She called for parliament to be recalled immediately.
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.— Jo Maugham QC (@JolyonMaugham) September 11, 2019
Mr Maugham added: "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."
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 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. https://t.co/EApHnB3fhQ— Ian Blackford (@IanBlackfordMP) September 11, 2019
Former Tory attorney general Dominic Grieve said the ruling in Edinburgh is a "serious indictment" on the government.
Mr Grieve said: "I think the judgment is a serious indictment on the government because, leaving aside whether the ruling in law is correct, it is quite clear that on the facts, the judges in Edinburgh concluded that the Government's explanations for proroguing Parliament were simply inaccurate and untrue."
He added: "We have been making the same point in the House of Commons."
After the Sun reported that Downing Street had attacked the ruling by Scottish judges as being "politically motivated", Scotland First Minister Nicola Sturgeon responded: "This is pitiful, pathetic and desperate from No10."
Ms Cherry added on Twitter: "Disgraceful slur on #judicialindependence. We chose the #Scottish courts because we are Scottish & they weren't on holiday. The is the Supreme Court of Scotland & a higher court than the court of 1st instance in England."
Downing Street has said ministers "absolutely respect" the independence of the judiciary following the ruling.
Sources in No 10 were quoted as saying that the MPs and activists who brought the case in the Court of Session in Edinburgh "choose the Scottish courts for a reason".
However the Prime Minister's spokesman said: "We absolutely respect the independence of the judiciary.
"I have spoken emphatically for the Prime Minister and for No 10."
The spokesman said ministers would respect the ruling of the Supreme Court, which is due to hear the government's appeal.
He rejected suggestions ministers had misled the Queen over the reasons given for seeking a prorogation.
"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