He had argued that shutting down parliament until October 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. Delivering the unanimous verdict of 11 judges, Supreme Court president Brenda Hale said "the decision to advise Her Majesty (Queen Elizabeth II) to prorogue was unlawful".
She said this was "because it had the effect of frustrating or preventing the ability of parliament to carry out its constitutional functions". As a result, the suspension was "void and of no effect", Hale said, adding: "Parliament has not been prorogued."
Bercow subsequently announced that he would reconvene the Commons at 11.30 am (1030 GMT) on Wednesday morning, while the upper House of Lords said it would return the same day. The judges "have vindicated the right and duty of parliament to meet at this crucial time to scrutinise the executive", Bercow said. A small group of protesters outside the court hailed the decision, with one, Gareth Daniels, telling AFP: "This is a great day for democracy."
Jeremy Corbyn, the leader of the main opposition Labour party, led calls for the prime minister to step down. "I invite Boris Johnson, in the historic words, to consider his position, and become the shortest serving prime minister there has ever been," he told his party's annual conference.
He brought forward his concluding speech to the meeting in Brighton, southern England, from Wednesday to Tuesday to allow him to be in Westminster. Some opposition MPs called for a confidence vote in Johnson, and Bercow indicated that he would allow time for this if a formal request were made. "I think Boris Johnson should resign," Scottish National Party leader Nicola Sturgeon told Sky News television.