Sitting in the high court, Hale said that Johnson’s decision to suspend 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.”

The court unanimously found that Johnson’s suspension was “void and of no effect,” meaning, essentially, that Parliament has not been suspended.

Opposition leaders said that they planned to reenter Westminster Palace on Wednesday and were awaiting a call from the John Bercow, the flamboyant speaker of the House of Commons, to bring them back into session.

“This is an absolutely momentous decision,” said Joanna Cherry, a Scottish politician who helped to launch the case in the Scottish courts.