Scottish judges delivered a hammer blow to Boris Johnson today by sensationally ruling Parliament was suspended illegally.
An Edinburgh court decided that prorogation was unlawful because the Prime Minister's intention had been to 'stymy' scrutiny from MPs.
The stage is now set for a titanic showdown at the Supreme Court on Tuesday - with the risk that the monarch could be dragged into the bitter Brexit war.
The judges suggested Mr Johnson misled the Queen about his motivations - concluding he had an 'improper purpose'.
Remainer politicians who brought the case claimed that the prorogation of Parliament - which happened in the early hours of Tuesday morning by Royal proclamation - was now null and void.
'We are calling on Parliament to be recalled immediately,' said the SNP's Joanna Cherry.
A UK Government spokesperson 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 dismissed a challenge against the planned prorogation at the Court of Session last Wednesday, saying it is for politicians and not the courts to decide.
But a panel of three judges in Edinburgh overturned that decision.
A summary of the judgement said: '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.'
The case is now set for the Supreme Court in London where it is expected to be heard alongside a similar case brought by campaigner Gina Miller.
Those who brought the case have been quick to celebrate the outcome.
Ms Cherry, one of the Scottish MPs who brought the challenge, tweeted: 'Huge thanks to all our supporters & our fantastic legal team who have achieved the historic ruling that #prorogation is #unlawful'
Jolyon Maugham QC, the anti-Brexit barrister who was second petitioner in the case, said the Supreme Court would hear the case next week.
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.'
Labour's Keir Starmer said: 'I welcome the Court's judgement. No one in their right mind believed Boris Johnson's reason for shutting down Parliament.
'I urge the Prime Minister to immediately recall Parliament so we can debate this judgement and decide what happens next.'
But Brexiteers voiced fury at the ruling.
Tory former MP Stewart Jackson said: 'The reputation of Parliament is as low as it can get now.
'The Scottish Court decision merely reinforces the narrative that the Establishment couldn't care less about the voters and will do all it can to overturn the democratic will of the people. Carry on. Tick tock.'