British Prime Minister Boris Johnson's decision to suspend parliament was unlawful, the Supreme Court ruled on Tuesday.
"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,” Supreme Court President Brenda Hale said.
The ruling by the court's 11 presiding judges follows an emergency three-day hearing last week.
"Parliament has not been prorogued," Hale said. "It is for parliament, and in particular the speaker and the lords speaker, to decide what to do next."
Downing Street said it was "currently processing the verdict".