Boris Johnson’s prorogation of parliament was “unlawful,” judges at the appeal court in Edinburgh, Scotland, have ruled.
The prime minister is now facing demands he recall the House of Commons immediately.
But the ruling does not cancel the suspension of the parliament as the government plans to appeal Tuesday’s ruling to the UK Supreme Court.
A group of around 70 parliamentarians had appealed against an earlier ruling by a judge that the prorogation was lawful.
But three judges of the Inner House, the supreme civil court in Scotland, overturned that decision.
Judge Lord Carloway said today: “We are of the opinion that the advice given by the government to her majesty the Queen to prorogue parliament was unlawful and that the prorogation itself was unlawful.”
The ruling comes a day after the prorogation took place in the early hours of Tuesday, with parliament now suspended for five weeks.
Opposition MPs have argued the longer than normal suspension of parliament was designed to try and prevent the Commons blocking a no-deal Brexit.
Shadow Brexit secretary Keir Starmer said the court ruling was “huge,” and vindicated Labour’s efforts to stop parliament being shut down.
Speaking at the TUC Congress in Brighton, he said: “I need to get back to parliament, to see if we can reopen the doors and hold Johnson to account.
“It was obvious to everyone that shutting down parliament at this crucial time was the wrong thing to do.
“The prime minister was not telling the truth about why he was doing it. The idea of shutting down parliament offended everyone across the country, and then they felt they were not being told the truth.”
Jolyon Maugham QC, the anti-Brexit barrister who was second petitioner in the case, said: “We believe that the effect of the decision is that parliament is no longer prorogued.”
Labour MP David Lammy said: “Let’s unlock the doors tomorrow. Or are you afraid of the UK’s sovereign parliament taking back control?”
The government said it was “disappointed” by the decision of the senior Scottish judges, adding proroguing parliament was ” legal and necessary.”
This story has been updated.