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.”