In a setback for Prime Minister Boris Johnson, Britain's Supreme Court ruled Tuesday that the suspension of Parliament was illegal.

The ruling is a major blow to the prime minister who had suspended Parliament for five weeks, claiming it was a routine closure.

Britain's highest court ruled that Johnson's government had actually shut Parliament to squelch debate on its Brexit policy.

Senior judge Brenda Hale said the suspension "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 Supreme Court on Tuesday said that parliamentarians could reconvene "as soon as possible."

"It is for parliament, and in particular the Speaker and the Lord Speaker, to decide what to do next. Unless there is some parliamentary rule of which we are unaware, they can take immediate steps to enable each House to meet," the ruling said.

The ruling was a unanimous decision by the court's 11 presiding judges.

Parliament was suspended, or prorogued in the British jargon, from Sept. 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.

House of Commons speaker John Bercow said Tuesday in response to the ruling that Parliament must "convene without delay."

Bercow, who has been highly critical of Johnson's decision, said he would be consulting party leaders "as a matter of urgency."