The court, in a short ruling, said that the purpose of the Prime Minister’s move was to unlawfully “stymie” Parliament. The unanimous decision Wednesday by a panel of Edinburgh appeal judges will set up a showdown in the U.K. Supreme Court, which will take up the issue next week.

“My message to Boris Johnson is you are playing fast and loose with the law,” Ian Blackford, a Scottish member of Parliament, said in a tweet. “You have acted in an anti-democratic manner and need to respond by recalling Parliament.”

While Johnson had suffered several political defeats in Parliament, until now he had fared better in court with victories in London and at a lower court in Edinburgh. The threat of prorogation galvanized Labour politicians and a group of Conservative rebels into passing a bill requiring the prime minister to push back the date when the U.K. leaves the European Union if he can’t get a deal in Brussels.

A group of more than 70 lawmakers had argued that the prime minister’s move — which took effect Monday night — was unconstitutional because it curtailed debate in the run-up to the deadline for Britain’s exit from the EU.

Devereux Chambers Tax Lawyer Jolyon Maugham Portraits
Jolyon Maugham Photographer: Jason Alden/Bloomberg
“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,” Jolyon Maugham, the attorney spearheading the Scottish case, said after the ruling. “I am pleased that Scotland’s highest court agrees.”

The three judges didn’t issue an interim order on whether Parliament can sit in the meantime, but Maugham said he believed that lawmakers could convene immediately.

Johnson’s move to prorogue was unlawful, the judges said, because it was motivated by the “improper purpose” of stymieing Parliament.

“This was an egregious case of a clear failure to comply with generally accepted standards of behavior of public authorities,” Judge Philip Brodie said in the ruling.

The government said it was disappointed with the decision and would appeal the case to the top court.

“The U.K. government needs to bring forward a strong domestic legislative agenda,” the government said in a statement. “Proroguing Parliament is the legal and necessary way of delivering this.”

The legal action now moves to a hearing before the country’s top judges on Sept. 17, where the Supreme Court will need to reconcile a different judgment from London. A court in Northern Ireland is also set to rule on the matter on Thursday.

(Updates with judges’ ruling from fourth paragraph.)
To contact the reporters on this story:
Alastair Reed in Edinburgh at [email protected];
Jonathan Browning in London at [email protected]

To contact the editor responsible for this story:
Anthony Aarons at [email protected]