LONDON — Britain’s highest court dealt a serious blow Tuesday to Prime Minister Boris Johnson, ruling that his controversial decision to suspend Parliament was unlawful, in a landmark judgment that could have implications for Johnson’s future and the country’s departure from the European Union.

Opposition politicians immediately called on Johnson to quit. Johnson indicated no intention to do so.

Labour Party leader Jeremy Corbyn said the court’s decision demonstrates Johnson’s “contempt for democracy and abuse of power.”

“He thought he could do whatever he liked — just as he has always done, all his life,” Corbyn said. “He thinks he’s above us all.”

Joanna Cherry, a Scottish politician who helped launch the case in the Scottish courts, called the ruling “absolutely momentous.” Johnson’s position, she added, “is untenable. He should have the guts for once to do the decent thing and resign.”

In New York, where he was addressing the United Nations General Assembly on Tuesday, Johnson told reporters he “disagreed profoundly” with the court decision but added, “We respect the judiciary in our country.”

He vowed to press ahead — and again promised to deliver Brexit by the end of October.

Johnson has maintained that lawmakers — and now the courts — are trying to block Brexit and undermine him in withdrawal negotiations with the European Union.

Britain's Supreme Court ruled Sept. 24 Prime Minister Boris Johnson acted unlawfully when he advised Queen Elizabeth to suspend Parliament weeks before Brexit. (The Washington Post)

But in one of the highest-profile cases to come before Britain’s Supreme Court, 11 justices ruled unanimously that it was Johnson who was obstructing democracy.

The court said Johnson’s decision to suspend Parliament for five weeks ahead of the Oct. 31 Brexit deadline “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 noted that typically such breaks are measured in days, not weeks, and that this unusually long suspension came at a critical moment when Britain’s future was about to be determined.

Speaking at the U.N. General Assembly, President Trump said that Boris Johnson could survive a U.K. Supreme Court ruling he had suspended Parliament illegally. (The Washington Post)

The court judged that the suspension was “void and of no effect.” Parliament will reconvene Wednesday.

Legal scholars in Britain said the judgment was historic — on par with Marbury v. Madison, one of the U.S. Supreme Court’s most famous cases, which established the principle of judicial review.

The British Supreme Court rejected the government’s argument that suspending Parliament was a political and not a legal matter. The court was emphatic that the executive does not have unfettered power.

Another plaintiff in the British court proceedings, Gina Miller, a transparency activist, told Sky News: “This case is much, much bigger than Brexit. It is about the prime minister abusing his power and giving the queen illegal advice.” 

House of Commons Speaker John Bercow, a flamboyant character in the Brexit drama and vocal advocate for the rights of lawmakers, called the high court’s decision “unambiguous” and “unqualified.”

Bercow foreshadowed what is likely to be a freewheeling session for the chamber Wednesday, with lawmakers allowed to ask “urgent questions” of Johnson’s ministers and take part in “emergency debates.”

What happens next is anyone’s guess. Lawmakers spent Tuesday spinning possible scenarios. Some were pushing for a no-
confidence measure against Johnson. The prime minister already has lost his majority in Parliament. But it was unclear whether there were sufficient numbers to force him out.

Others thought that Johnson might try to suspend Parliament again or that new elections should be quickly staged.

Johnson said he would continue to seek an exit deal with E.U. leaders. He doubled down and promised that Britain would leave with or without a withdrawal agreement, even though Parliament has passed — and the queen signed — a bill forbidding Johnson to take Britain out of Europe without a deal, making another court case possible, too.

While Johnson’s options are limited, he has a few more cards to play. Even if his vision for Brexit is foiled and he is challenged in a snap election, the mop-headed prime minister remains a formidable campaigner and a popular if polarizing figure.

A YouGov poll published last week found that Johnson was still more popular than Labour’s Corbyn. Thirty-eight percent of the British public indicated a “favorable” view of Johnson, and 54 percent said their view was “unfavorable.” By comparison, the poll found that 21 percent had a positive view of Corbyn, and 70 percent had a negative view.

Still, the Supreme Court ruling is a deep wound. Johnson’s move to suspend Parliament was a bold and unprecedented one — and it backfired in spectacular fashion. 

John Major, a former prime minister and fellow Conservative who submitted testimony against Johnson, said he hopes the ruling “will deter any future prime minister from attempting to shut down Parliament, with the effect of stifling proper scrutiny and debate, when its sitting is so plainly in the national interest.”

Major added, “No prime minister must ever treat the monarch or Parliament in this way again.”

Joelle Grogan, a senior lecturer in law at Middlesex University, said she watched the judgment in a room full of academics, who were stunned by the decision. “It’s the rewriting of an unwritten constitution,” she said.

“The truth of Brexit is that it’s exposing questions to the courts that they’ve never been exposed to before,” Grogan said. “We’re now asking them basic questions: What is Parliament? What is government? What is the separation between the two . . . and when there is a clash of law and politics, who gets to make the big decisions?”

Grogan said that one remarkable aspect of this case was its speed. Cases often take years to weave through the lower courts before they arrive at the highest court in the land, she said.

In this instance, Johnson announced at the end of August that he planned to shutter Parliament. Less than a month later, the out-of-session Supreme Court was called in to consider combined legal challenges on appeal. Tuesday’s ruling followed three days of hearings last week.

In New York on Tuesday, Johnson’s schedule included a bilateral meeting with President Trump, who is also facing a fresh political scandal this week.

Speaking with reporters, the U.S. president was asked about his reaction to the British court decision. 

“I had no reaction,” Trump said. “I just asked Boris, and, you know, to him it’s another day in the office.”

Johnson followed that with: “Tomorrow is another day in Parliament.”