Boris Johnson’s suspension of the UK Parliament is unlawful, Scotland’s highest civil court has ruled.
A panel of three judges at the Court of Session found in favour of a cross-party group of politicians who were challenging the prime minister’s move.
The judges said the PM was attempting to prevent Parliament holding the government to account ahead of Brexit.
The UK government said it will appeal against the ruling to the Supreme Court in London.
The Court of Session decision overturns an earlier ruling from the court, which said last week that Mr Johnson had not broken the law.
But it is currently unclear what impact the judgement will have on the current suspension of Parliament – a process known as proroguing – which started in the early hours of Tuesday.
MPs are not scheduled to return to Parliament until 14 October, when there will be a Queen’s Speech outlining Mr Johnson’s legislative plans. The UK is due to leave the EU on 31 October.
Mr Johnson has previously insisted that it was normal practice for a new government to prorogue Parliament, and that it was “nonsense” to suggest he was attempting to undermine democracy.
But the Court of Session judges said they were unanimous in their belief that Mr Johnson was motivated by the “improper purpose of stymieing Parliament”, and that he had therefore misled the Queen.
They added: “The Court will accordingly make an Order declaring that the prime minister’s advice to HM the Queen and the prorogation which followed thereon was unlawful and is thus null and of no effect.”
The group of more than 70 largely pro-Remain MPs and peers behind the legal challenge were headed by SNP MP Joanna Cherry, who called for Parliament to be immediately reconvened following the ruling.
She added: “We feel utterly vindicated and I would be confident that the UK Supreme Court will uphold this decision.”
The parliamentarians appealed to the Inner House of the Court of Session after their original challenge to the suspension of Parliament was dismissed by judge Lord Doherty last week.
Lord Doherty said Mr Johnson had not broken the law by proroguing Parliament, and that it was for MPs and the electorate to judge the prime minister’s actions rather than the courts.
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.
One of the three judges, Lord Brodie, said: “This was an egregious case of a clear failure to comply with generally accepted standards of behaviour of public authorities.
“It was to be inferred that the principal reasons for the prorogation were to prevent or impede Parliament holding the executive to account and legislating with regard to Brexit, and to allow the executive to pursue a policy of a no-deal Brexit without further Parliamentary interference.”
And Lord Drummond Young said that the UK government had failed to show a valid reason for the prorogation, adding: “The circumstances, particularly the length of the prorogation, showed that the purpose was to prevent such scrutiny.
“The only inference that could be drawn was that the UK government and the prime minister wished to restrict Parliament.”
The judges will release their full findings on Friday.