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 judges 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.”
The PM says he “strongly disagrees” with the ruling but will “respect” it.
A raft of MPs have now called for the prime minister to resign.
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 and the suspension was far longer than necessary for a Queen’s Speech.
Delivering its conclusions, the Supreme Court’s president, Lady Hale, said: “The decision to advise Her Majesty to prorogue Parliament was unlawful because it had the effect of frustrating or preventing the ability of Parliament to carry out its constitutional functions without reasonable justification.”
Lady Hale said the unanimous decision of the 11 justices meant Parliament had effectively not been prorogued – the decision was null and of no effect.
She added that it was important to emphasise the case was “not about when and on what terms” the UK left the EU, but about the decision to suspend Parliament.
Speaker of the Commons John Bercow said MPs needed to return “in light of the explicit judgement”, and he had “instructed the House of Commons authorities to prepare… for the resumption of business” from 11:30 BST on Wednesday.
He said prime minister’s questions would not go ahead, but there would be “full scope” for urgent questions, ministerial statements and applications for emergency debates.