A ruling had initially been made last Wednesday by Judge Lord Doherty, who said it was for politicians - not the courts - to decide on whether the suspension of Parliament was lawful.
A group of around 70 parliamentarians appealed and today three judges of the Inner House, the supreme civil court in Scotland, disagreed with Lord Doherty's ruling and said prorogation was "null and of no effect".
Labour's Barry Sheerman, who represents Huddersfield, tweeted: "We should recall Parliament immediately. I am heading into Parliament as I did yesterday."
Rachel Reeves, Labour MP for Leeds West, said: "This ruling proves that Boris Johnson's disgraceful attempt to silence Parliament was not just a dangerous threat to democracy - it was illegal too. I hope to be back in Parliament representing my constituents ASAP, whether Boris Johnson likes what I have to say or not."
While Rachael Maskell, Labour MP for York Central, added: "Let’s get Parliament open and [the] PM held to account over this shambles."
The Government said it is "disappointed" by the decision and would be appealing to the UK's Supreme Court.
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 9 and 12 September until 14 October was unlawful because it had the purpose of stymying Parliament."It continued: "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."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.Campaigners said their understanding is that Parliament can now reassemble if it so wishes.The ruling comes a day after the prorogation took place in the early hours of Tuesday, with Parliament now suspended for five weeks.
The campaigners included SNP MP Joanna Cherry QC, Liberal Democrat leader Jo Swinson, Labour MP Ian Murray and anti-Brexit barrister Jo Maugham, of the Good Law Project.Ms Cherry tweeted: "Huge thanks to all our supporters & our fantastic legal team who have achieved the historic ruling that #prorogation is #unlawful #Cherrycase #Brexit #StopTheCoup."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."Shadow Brexit secretary Keir Starmer said the court ruling was "huge", and vindicated Labour's efforts to stop Parliament being shut down.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."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."
Liberal Democrat shadow Brexit Secretary Tom Brake said: "The decision today is highly embarrassing for Boris Johnson and his government. The implications of this decision made by the Scottish Courts should not be underestimated - the shutdown of Parliament has been found unlawful and the government must now act accordingly. "The prorogation of Parliament was never more than a power grab. 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 "Boris Johnson is trampling on the very values and principles the United Kingdom was founded upon. The Liberal Democrats will continue to fight both in Parliament and in the courts to protect our democracy and stop a no-deal Brexit".