"This is a valid ruling today by the court", Paul Sweeney, one of the politicians who had put their name to the legal challenge, told Al Jazeera.
"The Supreme Court will now rule on the matter on Tuesday and the government will abide by that decision", said a government spokesperson.
"In this context, his own proroguing of Parliament has come back to hoist him on his own petard, as MPs will not return to the House until October 14, so that is the earliest point at which they will be able to vote for an election (with the requisite two-thirds of MPs having to vote in favour as required under the Fixed-Term Parliament Act)".
Last week, his government lost control of Parliament to an informal coalition of the main opposition and Conservative rebels, who then passed a bill on Monday to block a "no-deal" Brexit within the current deadline.
But the prorogation now means Boris Johnson has five weeks to negotiate an exit deal with the European Union - an option many believe is Boris' only hope of leaving the European Union by the October 31 deadline.
But EU officials say the United Kingdom has made no concrete new proposals.
It found that Johnson's advice to Queen Elizabeth II to prorogue parliament "was unlawful because it had the goal of stymying parliament", a summary judgement said.
- When will the Supreme Court make a ruling? .
"Huge thanks to all our supporters & our fantastic legal team who have achieved the historic ruling that #prorogation is #unlawful", Scottish National Party lawmaker Joanna Cherry said on Twitter.
"We believe that the effect of the decision is that parliament is no longer prorogued", he said following the court's decision.
But the three Inner House judges said they disagreed with Lord Doherty's ruling because this particular prorogation had been a "tactic to frustrate Parliament" rather than a legitimate use of the power.
The language used by three judges, including the head of the Scottish judiciary Lord Carloway, is scathing: "This was an egregious case of a clear failure to comply with generally accepted standards of behaviour of public authorities".
"This was because of the length of time and the documents released showing the political discussions around prorogation that took place".
While the full judgement will be handed down later in the week, the judges had stark words for the British government in a summary of their findings.
The UK government said it was "disappointed" by the decision and would be appealing to the UK's Supreme Court.
In a statement, the government said it "needs to bring forward a strong domestic legislative agenda".
Its ruling was at odds with the High Court in England which said that the shutdown was not illegal even if it was motivated by the desire for "political advantage".
"It's absolutely essential to our constitution that the relationship between the prime minister and the Queen is one of utmost confidentiality and the utmost good faith - essential!", Mr.
"I call on him to recall Parliament".
Mr Benn told Sky: 'We need Parliament sitting now more than ever and it is unacceptable that Boris Johnson sent us away.
Britain's former Attorney General Dominic Grieve was reported by the Daily Telegraph to have called on Johnson to resign "if he misled the Queen". Parliament has already passed a law requiring the Prime Minister to seek an extension of the October 31 deadline if he hasn't reached a deal by October 19.
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