Britain's Supreme Court ruled on Tuesday that a decision by Prime Minister Boris Johnson to suspend parliament in the run-up to Brexit was "unlawful", saying it was "void and of no effect".
The 11 judges of the country's highest court were unanimous in their verdict, which they said meant parliament could now immediately reconvene.
"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.
"Parliament has not been prorogued. This is the unanimous judgment of all 11 justices," she added. "It is for parliament, and in particular the speaker and the lords speaker, to decide what to do next."
The decision was a stunning blow to Johnson that sparked immediate calls for him to resign.
Parliament was suspended, or prorogued, from Sep 10 to Oct 14. The prorogation was approved by Queen Elizabeth, Britain's politically neutral head of state, acting on the advice of the prime minister as she is required to do under the country's complex, uncodified constitution.
Johnson had argued that shutting down parliament until Oct 14 was a routine move to allow his new government to set out a new legislative programme.
But critics accused him of trying to silence MPs ahead of Britain's scheduled exit from the European Union on Oct 31 - the terms of which remain unclear.