Boris Johnson's decision to suspend parliament for five weeks was unlawful, the Supreme Court has decided.
A panel of 11 justices at the highest court in the UK have unanimously voted that the prime minister's advice to the Queen to prorogue parliament until October 14 was unlawful.
The judges, led by the court's president Lady Hale, heard appeals over three days after legal challenges in England and Scotland that reached different conclusions.
The court also found the prorogation was "void and of no effect" - meaning parliament has not been suspended.
Announcing the result, Lady Hale said: "The court is bound to conclude, therefore, that 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."
At the High Court in London, the Lord Chief Justice Lord Burnett and two other judges rejected a challenge against the prime minister's prorogation move by campaigner and businesswoman Gina Miller.
But in Scotland, a cross-party group of MPs and peers won a ruling from the Inner House of the Court of Session that Mr Johnson's prorogation decision was unlawful because it was "motivated by the improper purpose of stymieing parliament".
The prime minister advised the Queen on August 28 to prorogue parliament for five weeks and it was suspended on September 9.
Mr Johnson claimed the five-week suspension was to allow the government to set out a new legislative agenda in a Queen's Speech when MPs return to parliament.
Labour leader Jeremy Corbyn has called for Boris Johnson to "consider his position" following the Supreme Court decision.
Brexit Party leader Nigel Farage called for the prime minister's chief adviser Dominic Cummings to be removed from his role following the Supreme Court's decision.
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