However others said it was for the government to recall Parliament, which it said it would not do before appealing the decision to the Supreme Court early next week.

Labour MPs immediately called for the government to “unlock the doors” of the House of Commons within 24 hours.

Shadow Brexit secretary Kier Starmer, a lawyer turned MP, welcomed the judgment.

"No one in their right mind believed Boris Johnson's reason for shutting down Parliament," he said. "I urge the Prime Minister to immediately recall Parliament so we can debate this judgment and decide what happens next."

Former Conservative Attorney-General Dominic Grieve said if the court's decision was upheld on appeal the Prime Minister should resign "very swiftly" or would find himself in an untenable position in parliament and with his own colleagues.

The relationship between the PM and the Queen must be one of "absolute faith", Grieve said. "The Prime Minister has a duty of total candour towards the Queen when asking her to discharge her functions, it's essential otherwise our constitution can't work."

Two weeks ago the Johnson government, through the Privy Council, obtained an order from the Queen to prorogue parliament for five crucial weeks in the lead-up to Brexit. The Prime Minister said it was in order to begin a fresh parliamentary session and present his government’s new legislative agenda.

However, on Wednesday morning, UK time, the Scottish Court of Session ruled the order “was unlawful because it had the purpose of stymying Parliament”.

In a summary of a full judgment to be delivered on Friday the three judges ruled this was an “improper purpose”.

Parliamentary scrutiny of the executive was a “central pillar of the good governance principle enshrined in the constitution,” Lord President Lord Carloway said. “This followed from the principles of democracy and the rule of law.”

Partially redacted documents exhibiting some of the government’s deliberations, dating back to mid-August, had demonstrated the “true reason for the prorogation”, he said.

Decisions of the Queen on advice from the Prime Minister cannot usually be reviewed, but there is a rare exception if the advice is unlawful.

“The particular prorogation that had occurred, as a tactic to frustrate Parliament, could legitimately be established as unlawful,” Lord Brodie said. “This was an egregious case of a clear failure to comply with generally accepted standards of behaviour of public authorities.”

Lord Drummond Young said parliamentary scrutiny of executive action had “fundamental constitutional importance”.

The circumstances, particularly the length of the prorogation – one of the longest in a century, and weeks longer than usual – “showed that the purpose was to prevent such scrutiny”.

The Court said it would make an order declaring that the Prime Minister’s advice to the Queen and the prorogation that followed “was unlawful and is thus null and of no effect”.

Jo Maugham, QC, who represented a group of MPs and activists challenging the prorogation, said he believed the effect of the court’s decision was that “Parliament is no longer prorogued”.

He said Parliament had been “treated as an inconvenience by the Prime Minister”.

But he emphasised the Supreme Court would have the final say.

Scottish National Party MP Joanna Cherry, who led the action, called it an “historic ruling”.

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.”