Boris Johnson's decision to suspend the British parliament this week for five weeks was unlawful and should be annulled, Scotland's highest court of appeal ruled on Wednesday in an embarrassing setback for the prime minister.

Parliament was prorogued on Monday until October 14, a move opponents argued was designed to thwart their attempts to scrutinize his plans for leaving the European Union and allow him to push through a no-deal Brexit on October 31.

"You cannot break the law with impunity, Boris Johnson," said Joanna Cherry, the Scottish National Party lawmaker who led the challenge.

"We are calling for parliament to be recalled immediately," she told Sky News after the unanimous verdict by three judges at Scotland's Court of Session.

Johnson's office said the government would appeal to the Supreme Court, the highest judicial body in the United Kingdom.

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It was not immediately clear what effect the ruling would have.

"Any decision to accelerate the meeting of Parliament during prorogation is a matter for the government," a spokeswoman for the Speaker of the House of Commons told the BBC.

Johnson announced on August 28 that parliament would be prorogued, saying the government wanted the suspension so it could then launch a new legislative agenda.

Opponents said the real reason was to shut down debate and challenges to his Brexit plans. The court was shown documents that showed Johnson was considering prorogation weeks before he formally asked Queen Elizabeth to suspend the legislature.

"The UK government needs to bring forward a strong domestic legislative agenda," a government spokesman said in response to Wednesday's ruling. "Proroguing parliament is the legal and necessary way of delivering this."

However, in an excoriating judgement, the Scottish judges ruled the principal reason for suspension was to stymie lawmakers and allow Johnson to pursue a no-deal Brexit policy.

"This was an egregious case of a clear failure to comply with generally accepted standards of behavior of public authorities," concluded one judge, Philip Brodie, according to a summary of the court verdict.

Judge James Drummond Young had determined that "the only inference that could be drawn was that the UK government and the Prime Minister wished to restrict Parliament," the summary said.

"The Court will accordingly make an Order declaring that the Prime Minister’s advice to HM (Her Majesty) the Queen and the prorogation which followed thereon was unlawful and is thus null and of no effect."

Cherry and other lawmakers, including some parliamentarians who were thrown out of Johnson's Conservative Party last week for rebelling over Brexit, said parliament should be recalled without delay.

One of the former Conservatives, Dominic Grieve, said if Johnson had misled the queen over the reasons for prorogation, he should resign.

"If that were to be the case that this had happened, Boris Johnson would find himself in an untenable position in parliament," Grieve told reporters.

Buckingham Palace declined to comment on the ruling.

A lower Scottishcourt had originally rejected the challenge and last Friday, London's High Court also dismissed a similar challenge by campaigners. An appeal in that case is due to be heard on September 17 at the Supreme Court.

Johnson, who took office in July, has promised to take Britain out of the EU on October 31 with or without a withdrawal agreement.

Before parliament was suspended, lawmakers forced through legislation which compels the prime minister to seek a three-month delay to Brexit on October 19 if no divorce agreement has been agreed to prevent a no-deal Brexit.

However, Johnson has ruled out asking the EU for any extension to the exit date.