LONDON — Three hundred years ago, after King James II had suspended Parliament and tried to rule alone, the pendulum of power swung in the British Isles. In the revolution that swept him aside, the monarchy, once supreme, ceded powers to lawmakers, gradually rendering itself an ornament in a system increasingly controlled by Parliament.
But the truce that evolved in the centuries that followed — between the government and monarchy on one side, and Parliament on the other — was threatened last month when Prime Minister Boris Johnson told the queen to send lawmakers home, starving them of precious time to make their own plans before Britain’s exit from the European Union.
So “egregious” was the overreach, a Scottish court said this week, that it did something no British court ever had before: It ruled that the prime minister had misled the public and unlawfully advised the queen on a suspension of Parliament, abusing some of the loftiest powers he had to clear his path of recalcitrant anti-Brexit lawmakers.
The decision triggered a Supreme Court hearing next week, setting up the most serious test in years of a British government’s power. Hanging in the balance is the fate of Mr. Johnson’s leadership, along with the delicate arrangement that has kept the monarch out of politics for so long.
The British courts, widely expected to stay out of the fight over suspending Parliament, have instead become the biggest story. The tabloids have been running exposés on Scottish judges’ love of France and jazz, while right-wing Conservative lawmakers have branded them part of an anti-Brexit “liberal elite.”
With that began an ugly new chapter in the fight for Brexit, one that has already turned some people in Britain against civil servants and lawmakers and may form the centerpiece of Mr. Johnson’s anti-establishment campaign at the next election.
The ruling, though, indicated that at least some judges will go further than scholars thought they would — and, in some cases, thought they should — to check Mr. Johnson’s hard-line tactics.
“All of these powers are technically available to the crown, but our Constitution works on the basis that they won’t be abused,” said Stephen Tierney, a professor of constitutional theory at Edinburgh University. “What we’re seeing recently is arguably the government, the executive, using the prerogative power in a way that it shouldn’t. This suggests that the crown is not playing this unwritten game properly.”
For a court to find the prime minister’s advice to Buckingham Palace unlawful was a major embarrassment for Queen Elizabeth II.
Under Britain’s constitutional traditions, suspending Parliament is a power available to the monarch alone. But she uses it solely on the government’s advice.
In this case, publicly at least, Mr. Johnson said he wanted to shut Parliament for five weeks at the height of the Brexit crisis simply to prepare for the launch of a new legislative agenda when the chamber reopened in mid-October.
Usually, though, that only takes a few days. The Scottish judges ruled that Mr. Johnson came to his decision to suspend Parliament in a “clandestine manner” and “specifically as a means to stymie any further legislation regarding Brexit.”
That spurred suggestions that Mr. Johnson had lied to the queen about his reasons for suspending Parliament. And it placed new strain on the convention that she should always take the prime minister’s word, a prerequisite of Britain’s uncodified Constitution.
“It’s a very uncomfortable position for the monarchy,” said Catherine Haddon, a senior fellow at the Institute for Government, a research institute. “I think it’s damaging, certainly, but you could also say it’s reinforcing the importance of keeping her out of politics.”
Mr. Johnson, for his part, has denied lying to the queen. Some scholars expect the Supreme Court to side with him next week by deciding that a prime minister’s advice to the queen on a question like this is a matter of politics, not law, and forbidden territory for the courts.
But a ruling against the prime minister would send a tremor through British politics that could cost Mr. Johnson his job. Some analysts suggested he would face enormous pressure to resign, with his leadership hanging on how many of his own Conservative colleagues were offended enough to try to force him out.
“It is absolutely central to our Constitution that the relationship between the prime minister and the queen is one of utmost confidence and the utmost good faith,” said Dominic Grieve, a lawmaker and former attorney general who was exiled from the Conservative Party by Mr. Johnson for blocking his Brexit plans. If Mr. Johnson had lied to the queen, Mr. Grieve said, “It would then be the moment for Mr. Johnson to resign, and very swiftly.”
The prime minister’s allies have struck back with an attack on the Scottish judiciary. In a television interview this week, one government minister, Kwasi Kwarteng, said repeatedly that “many people are saying that the judges are biased,” while another Johnson ally leveled a charge about the “liberal elite” using any means to defeat “the democratic decision of the people.”
Analysts saw in that the outlines of a no-holds-barred, anti-establishment campaign that Mr. Johnson seems eager to run if an election is called.
As for Parliament, lawmakers could immediately be reconvened if the Supreme Court rules that suspending the body was unlawful and nullifies it. But other lower courts — one in London, and another in Northern Ireland — have been more reluctant than the Scottish court to intervene, saying the suspension was not open to a legal challenge.
The Supreme Court could also split the difference, ruling that the prime minister misrepresented his reasons for wanting to suspend Parliament but that the chamber was still lawfully closed. Given the unusual circumstances, said Jim Cormack, a partner at Pinsent Masons, an Edinburgh law firm, the decision may not dramatically remake the courts’ view of executive power in future cases.
But the winners in the Scottish case hope it will establish some check on the prime minister’s power to suspend Parliament for as long as he pleases, and for any reason at all.
“If the courts are to be dissuaded from protecting Parliament in these circumstances, there’s really very little left of the British Constitution,” said Jolyon Maugham, a lawyer who helped bring the challenge in the Scottish courts.
The shutdown, scholars say, was a move against years of Parliament clawing back powers once reserved for the government. A 2011 law, for example, barred prime ministers from calling elections whenever they wanted, a change that has stymied Mr. Johnson in the Brexit fight. And a 2017 court case said the government needed lawmakers’ approval to trigger Britain’s withdrawal from the European Union.
But in the British system, the balance of power remains largely a matter of everyone following certain conventions, rather than any hard-and-fast rules.
And if there is one clear casualty from this particular fracas, it is the palpable erosion of that sense of common boundaries and customs.
“Neither side trusts the other to use the conventions that exist in our Constitution anymore,” Dr. Haddon said. “You end up in a situation where they’re all looking for more rigidity in what the rules of the game are.”