LONDON - In a major blow to Prime Minister Boris Johnson, Britain’s highest court ruled Tuesday that his decision to suspend Parliament for five weeks in the crucial countdown to the country’s Brexit deadline was illegal.

The unanimous, strongly worded Supreme Court judgment declared his order to suspend Parliament “void and of no effect.” The court found that Johnson acted to limit debate by lawmakers on Britain’s impending departure from the European Union in violation of Parliament’s constitutional role.

The landmark decision was quickly criticized by Johnson and prompted calls for him to quit from opposition leaders. The Conservative prime minister and Parliament have been at odds since he took power in July with the determination to take Britain out of the EU on Oct. 31 with or without a divorce deal.

“I strongly disagree with this decision of the Supreme Court . I have the upmost respect for our judiciary, I don’t think this was the right decision,” Johnson said in New York, where he was attending the U.N. General Assembly. “I think that the prorogation (suspension of Parliament) has been used for centuries without this kind of challenge.”

PM does not rule out trying to suspend Parliament again

Johnson did not rule out trying to suspend Parliament again. “As the law currently stands, the U.K. leaves the EU on Oct. 31 come what may, but the exciting thing for us now is to get a good deal. And that is what we are working on,” Johnson said. “And to be honest, it is not made much easier by this kind of stuff in Parliament or in the courts.”

House of Commons Speaker John Bercow welcomed the historic verdict and said Parliament would resume its business Wednesday morning. He said citizens are “entitled” to have Parliament perform its core constitutional duties, which include holding ministers to account and passing laws.

Bercow said there won’t be a Prime Minister’s Questions session in Parliament on Wednesday despite the fact that lawmakers were returning. Johnson’s office said, due to the ruling, the prime minister will fly back to London overnight from New York, earlier than planned, arriving by the time Parliament resumes.

The harsh tone of the court’s decision, and the unanimous vote of 11 Supreme Court judges, led many to say that Johnson can’t carry on. “His position is untenable and he should have the guts for once to do the decent thing and resign,” Scottish National Party legislator Joanna Cherry said outside the court.

Opposition Labour Party leader Jeremy Corbyn told his party conference that the court decision shows Johnson’s “contempt” for democracy and rule of law. He said Johnson should resign “and become the shortest-serving prime minister there’s ever been.”

“I invite Boris Johnson, in the historic words, to consider his position,” Corbyn told the party faithful in the southern city of Brighton.

Liberal Democrats leader Jo Swinson said the ruling made it clear that Johnson “is not fit to be prime minister.” She said he “acted unlawfully, tried to silence Parliament, tried to silence the voices of the people, because he does not want to be held to account ... for his disastrous Brexit policy.”

In this nation without a written constitution, the case marked a rare confrontation between the prime minister, the courts and Parliament over their rights and responsibilities. It revolved around whether Johnson acted lawfully when he advised Queen Elizabeth II to suspend Parliament for five weeks during a crucial time frame before the Oct. 31 Brexit deadline when Britain is scheduled to leave the 28-nation bloc.

Supreme Court President 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.”

She said the court’s decision means Parliament was never legally suspended and is technically still sitting. The decision Tuesday followed three days of hearings last week. The court rejected the government’s assertions that the decision to suspend Parliament until Oct. 14 was routine and not related to Brexit. Government lawyers claimed that under Britain’s unwritten constitution, it was a matter for politicians, not courts, to decide.

The government’s opponents argued that Johnson illegally shut down Parliament just weeks before its EU departure for the “improper purpose” of dodging lawmakers’ scrutiny of his Brexit plans. They also accused Johnson of misleading the queen, whose formal approval was needed to suspend the legislature.

Former Prime Minister John Major, a Conservative like Johnson, said after the Supreme Court ruling that he hoped it will prevent any future prime ministers from attempting to shut down Parliament in order to block it from doing its duty. “No prime minister must ever treat the monarch or Parliament in this way again,” he said.