LONDON — Prime Minister Boris Johnson was accused Tuesday of “unlawful abuse of power” in a high-profile court case about who runs the show in Britain: Parliament or the prime minister.

The Supreme Court is hearing challenges to Johnson’s contentious decision to suspend Parliament for five weeks — an unusually long break, coming just before Britain is scheduled to leave the European Union on Oct. 31.

Petitioners accuse Johnson of trying to curtail scrutiny by lawmakers. A Scottish court agreed, ruling last week that the five-week suspension was an “egregious” overreach, designed not as a brief pause between parliamentary sessions, as is customary, but to foil the legislature’s ability to shape Brexit plans.

An English court, however, dismissed a related case, determining that the issue was a political matter and not one for courts to decide.

The Supreme Court is hearing both cases on appeal.

In her introductory remarks, Supreme Court President Brenda Hale said the conflicting rulings demonstrate “that this is a serious and difficult question of law.”

Reflecting that significance, the formally out-of-session Supreme Court has been summoned for an emergency hearing, and the number of judges has been increased from the usual five or seven to 11. A total of 12 judges sit on the court, but they hear cases in odd numbers to avoid ties.

The court proceedings — being streamed live over three days — could have far-reaching implications for the balance of power between the legislature, the executive and the judiciary functions. The proceedings could also drag into play the role of the queen. And they could impact the direction of Brexit.

Hale emphasized, though, that the court was not seeking to determine “when and how the United Kingdom leaves the European Union.”

Dozens of protesters gathered outside the court, holding aloft placards that read “Don’t Silence our MPs” and “They Misled the Queen.”

In court on Tuesday, the government’s lawyer, Richard Keen, pointed to several previous occasions when Parliament was suspended for political purposes, including after the outbreak of World War I and at the beginning of the Great Depression. He argued that the executive was “entitled to do so” and that such suspensions, as political matters, “could not be declared unlawful by the courts.”

The judges pressed Keen on why the suspension had to last as long as five weeks. He replied that the government was trying to accommodate annual political party conferences, and suggested that lawmakers were only losing seven sitting days compared to their typical fall schedule.

David Pannick, a lawyer arguing the English case, said it was not proper for the government to suspend Parliament for such an “exceptionally long” time, frustrating the ability of the legislature to scrutinize the executive, legislate and hold debates.

“Under our system of constitutional law, Parliament is sovereign, and the executive is accountable to Parliament,” Pannick said. For the executive to impose a suspension “to evade control by Parliament stands the basic principles of constitutional law on their head.”

A judge suggested that Parliament could have held the government to account by calling a no-confidence vote.

The lawyer replied that Parliament wanting, or not wanting, to hold a no-confidence vote was a separate issue from the legality of what the prime minister did.

Legal analysts said it was too early to say which way the court might go.

“It’s such unchartered territory, there’s such strong arguments on both sides,” said Joelle Grogan, a senior lecturer in law at Middlesex University. “If I had tea leaves, I would not be able to read them.”

Asked who had the better day, she said Pannick had the “harder argument” to make, but Keen had “more difficult questions and challenges from the bench.”

Pannick is representing Gina Miller, a business executive and transparency activist. Miller and her legal team won a similarly momentous 2017 case in which the high court ruled the government must get Parliament’s approval before “invoking Article 50” and starting the countdown to Brexit.

Writing in the Independent newspaper on Monday, Miller argued that the current case: “is so much more important even than Brexit. It is about how we are governed, about preserving our ancient democratic freedoms and trying at all costs to stop a dangerous precedent.”

Miller charged Johnson with wanting to “put himself above the law” and suggested that if he’s allowed to have his way, future prime ministers could bring back the poll tax or reintroduce the death penalty without any input from lawmakers.

Johnson, in an interview with the BBC on Monday, disparaged “all this mumbo jumbo about how Parliament is being deprived the opportunity to scrutinize Brexit.”

“What a load of claptrap,” he said, arguing that the suspension was necessary to “set our ambitious agenda for the country.”

Johnson has denied a further contention of the Scottish court: that his government was misleading about its motivations for the suspension, including perhaps to the monarch.

“Absolutely not,” the prime minister responded last week to a reporter who asked if he had lied to Queen Elizabeth II.

The queen must approve any parliamentary suspension, but by custom she must abide by the advice of her prime minister.

Parliament was shut down in the wee hours of Sept. 10, following chaotic scenes on the floor of the House of Commons. It is scheduled to remain shuttered until Oct. 14, unless the court rules against Johnson and orders lawmakers back sooner.

Britain’s Supreme Court is nowhere near as interventionist as the Supreme Court in the United States. But this case is testing the boundaries of the executive and where the courts can weigh in.

“This is really Constitutional Law 101,” said Catherine Barnard, a professor of European law at Cambridge University. “First year constitutional law students coming in don’t need to read textbooks, they just need to read Twitter.”

Britain doesn’t have a written constitution. It relies on the rule of law, precedent and convention. And Johnson is a convention-busting prime minister.

Barnard said that those in the government’s corner will argue that the case should stay “in the political domain . . . and it means the courts aren’t caught up in political controversy.” 

Those in the opposing corner will argue that “surely no government power should be unlimited,” she said.

“It’s a fight almost to the death between the executive and the legislature,” said Jo Maugham, a lawyer and one of the driving forces in the Scotland case brought by 78 lawmakers.

Maugham voiced fears about another possible suspension shortly after Parliament returns on Oct. 14. In theory, he said, Johnson could suspend Parliament for the entire term and “govern untrammeled by the legislature.”

“The prorogation is unusual, so we are seeing unusual litigation flowing from it,” said Raphael Hogarth, a scholar with the Institute for Government think tank. “I think it’s inevitable: Once nobody feels like they know the rules of the game in Westminster anymore they will start turning to the courts.”

Johnson has defended the independence of British judges, saying the judiciary is “one of the great glories of our constitution — they are independent.”

He has also said he will abide by the law. But in the next breath he hints that he may not. 

Although Parliament passed a law demanding that he request a Brexit delay if a withdrawal deal hasn’t been agreed by the middle of October, Johnson insists he will not seek any further delays.

“We will obey the law,” he told the BBC on Monday. “But we will come out — and — we will come out, I should say, on 31 October.”