Britain's Prime Minister Boris Johnson addresses the Climate Action Summit in the United Nations General Assembly at U.N. headquarters in New York on Monday. Credit: Jason DeCrow | AP

LONDON — Britain’s highest court dealt a major blow on Tuesday to Prime Minister Boris Johnson, ruling that his controversial decision to suspend Parliament was unlawful, in a landmark decision that will have immediate implications for Britain’s departure from the European Union.

In one of the most high-profile cases to come before Britain’s Supreme Court, the 11 judges ruled unanimously that Johnson had not acted lawfully in shuttering Parliament.

The court ruled that Johnson’s decision to ask the Queen Elizabeth II to suspend Parliament essentially frustrated the ability of lawmakers to do the business of democracy, including debating Johnson’s plans for Brexit.

The ruling was a brutal one for Johnson, asserting that his move to suspend the Parliament was political maneuver and suggesting that he might have misled the queen.

Johnson has said he will not resign.

The ruling follows a three-day court hearing last week at the highest court, which was hastily convened to weigh contrasting judgments from English and Scottish courts on Johnson’s decision to suspend Parliament until mid-October.

The high-profile legal case concerns Johnson’s decision to ask the queen to suspend Parliament from Sept. 10 to Oct. 14, which the prime minister said was needed to introduce his “exciting” new legislative agenda. The queen agreed with the advice of her prime minister, as is customary.

Johnson’s critics say that the break — the longest since 1945 — was an attempt to thwart lawmakers’ ability to scrutinize the government’s Brexit plans ahead of Britain’s scheduled departure from the bloc on Oct. 31.

Johnson, who is in New York for the U.N. General Assembly, told the Daily Telegraph on Monday that he would not resign if the court ruled against him, insisting that he was correct to suspend Parliament so the government could focus on its domestic agenda. “It is absolutely absurd to be totally fixated on Brexit,” he said.

Johnson also broke into French at one point, saying that people should give him a break.

“Donnez-moi un break, is my message to those who say there will be no Parliamentary scrutiny. It is absolute nonsense,” he said.

Last week, 11 justices on Britain’s Supreme Court listened to appeals from two cases with contrasting judgments. A Scottish court found that Johnson’s decision to suspend Parliament was unlawful “because it had the purpose of stymieing Parliament” ahead of Brexit. An English court dismissed a similar case saying that it was political and not a matter for the courts.

In court last week, lawyers for the government argued that the case was not one for the courts, while lawyers for the claimants posited that the executive should not have unfettered power.

In one of the more surreal moments, a former Conservative prime minister criticized the current Conservative prime minister about his motives for suspending Parliament. In a written submission, John Major, a pro-European who was prime minister from 1990-1997, accused Johnson of being motivated by “political interest.”

The case is unusual on many fronts, not least because it’s testing the limits of Britain’s largely uncodified constitution. It’s normal for a prime minister to suspend Parliament, but the break is usually measured in days, not weeks.

“The truth of Brexit is that it’s exposing questions to the courts that they’ve never been exposed to before,” said Joelle Grogan, a senior lecturer in law at Middlesex University. “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?”

She said that the courts were “navigating strange constitutional waters, with no map.”

Grogan said one remarkable aspect of the case has been the speed with which it was pulled together. Cases that come before the Supreme Court often take years to weave through the lower courts before they arrive at the highest court in the land, she said. Johnson announced he was suspending Parliament only at the end of August.

Analysts had said that it was unclear what would happen next if the Supreme Court ruled against Johnson on Tuesday. Will the court leave it to Parliament or the executive to decide? Or could they issue a mandatory order recalling Parliament?

The majority of lawmakers in Parliament oppose leaving the EU without a deal, which the government’s own analysis says could affect the supply of medicines and some fresh foods. A cross-party rebel alliance has passed a law that would mandate Johnson to seek a three-month delay if no deal is struck by Oct. 19. Johnson says that Britain is leaving the bloc on Oct. 31, “do or die.”

In court last week, the government’s lawyers said that they would abide by the Supreme Court’s ruling, but they didn’t rule out suspending Parliament for a second time.