Boris Johnson’s decision to suspend Parliament was unlawful, the Supreme Court has ruled.
Mr Johnson suspended – or prorogued – Parliament for five weeks earlier this month, but the court said it was wrong to stop MPs carrying out duties in the run-up to Brexit on 31 October.
Supreme Court president Lady Hale said “the effect on the fundamentals of democracy was extreme.”
Downing Street said it was “currently processing the verdict”.
Mr Johnson argued he wanted to carry out the prorogation ahead of a Queen’s Speech so he could outline his government’s new policies.
But critics said he was trying to stop MPs from scrutinising his Brexit plans.
A raft of MPs have now called for the prime minister to resign and for Parliament to return as soon as possible.
Labour leader Jeremy Corbyn said the ruling showed Mr Johnson’s “contempt for democracy”, adding: “I invite Boris Johnson, in the historic words, to consider his position.”
The ruling was made after a three-day hearing at the Supreme Court last week which dealt with two appeals – one from campaigner and businesswoman Gina Miller, the second from the government.
Mrs Miller was appealing against the English High Court’s decision that the prorogation was “purely political” and not a matter for the courts.
The government was appealing against the ruling by Scotland’s Court of Session that the prorogation was “unlawful” and had been used to “stymie” Parliament.
The court ruled in favour of Mrs Miller’s appeal and against the government’s.