LONDON • Britain's all-consuming debate over Brexit has dragged another of its respected institutions into uncharted territory, as the Supreme Court struck down Prime Minister Boris Johnson's suspension of Parliament, an extraordinary intervention by the judiciary into a political dispute.

The unanimous decision, handed down on Tuesday, is an unalloyed defeat for Mr Johnson, and will propel Britain into a fresh round of political turmoil.

But it is even more significant for what it says about the role of the country's highest court, which has historically steered clear of politics.

By ruling that Mr Johnson acted unlawfully - and doing so in such stark language - the court asserted its right to curb a government that obstructed Parliament's ability to "carry out its constitutional functions as a legislature and as the body responsible for the supervision of the executive".

The sweeping nature of the ruling surprised legal scholars. "It is absolutely stunning," said Dr James Grant, a senior lecturer in law at King's College London. "It has developed long-established common law principles in a new direction."

Dr Grant said the ruling reaffirmed Parliament's right to hold the government to account in the face of a Prime Minister determined to pull Britain out of the European Union next month, regardless of the views of lawmakers.

Other scholars expressed regret that the court had felt compelled by Mr Johnson's actions to cast off a long tradition of legal restraint.

"It is a great pity that it should have been necessary for the court to intervene," said Mr Jonathan Sumption, who served as a Supreme Court justice from 2012 to 2018.

"But if the government takes an axe to the political convention and there are no rules, then there is a complete void in which the executive can act however it likes."

At issue was whether Mr Johnson, in suspending Parliament for five weeks in the middle of a dispute over Britain's departure from the European Union, had stymied the ability of lawmakers to have a say in that process.

The court, in upholding a previous ruling by a Scottish high court, judged that he had. Not only did the court declare the Prime Minister's action unlawful, it also declared the order itself, which Queen Elizabeth II issued at Mr Johnson's request, "unlawful, void and of no effect".

The request, said court president Brenda Hale, might as well have been a "blank sheet of paper".

Professor Stephen Tierney, a professor of constitutional theory at Edinburgh University, said it was astonishing that the court had ruled decisively that it "can review something as fundamental as that, done by Her Majesty, as unlawful".

"The court is getting involved in what was largely seen as the internal workings of Parliament and its supreme power," Prof Tierney said. "That, in itself, is unprecedented in the UK system. It is far more similar to the sort of judicial review you would get in a country with a codified Constitution."

In the United States, the courts have intervened regularly to challenge actions of the Trump administration, such as its ban on visitors from predominantly Muslim countries. The Supreme Court routinely exercises judicial review by actively interpreting the US Constitution.

Britain, however, relies on a partly unwritten set of traditions and conventions that have treated a sovereign Parliament as the supreme power in the land.

Once the courts venture into the political sphere and begin to pass judgment on Parliament's actions, some legal analysts say, there is no going back.

Lawyers for the government had argued that the courts had no jurisdiction in the case, an argument that an English lower court had accepted. But the Supreme Court decided otherwise. "The conduct of government by a prime minister and Cabinet, collectively responsible and accountable to Parliament, lies at the heart of Westminster democracy," Justice Hale declared.

She dismissed the government's legal case for suspending Parliament, which had been laid out by Mr Johnson's legislative director, Ms Nikki da Costa. And she used withering language to describe the effect of the court's ruling on the formal order to prorogue.

It would now be as if officials had "walked in with a blank sheet of paper", Justice Hale said.

Legal scholars said the most striking passage of the ruling was the court's decision to effectively cement a new constitutional principle: Parliamentary accountability, or the right of lawmakers to scrutinise the workings of government in what had previously been legally uncodified ways.

While legal experts said the decision had the potential to set a precedent for further judicial activism, they noted that it came in exceptional circumstances: A government that appeared ready to flout any and all rules that threatened its plans to take Britain out of the European Union by the deadline of Oct 31.

"We will hear a lot people alleging that this is a power grab from the Supreme Court," Dr Grant said.

"It is really not. This is not about the Supreme Court diminishing the power of elected politicians. It is about ensuring that the elected politicians are able to fulfil their constitutional functions."

The decision is likely to anger segments of British society for whom the Supreme Court, an institution founded only in 2009, has been largely anonymous until today.

"The courts see it as revealing the Constitution, but many people will see it as inventing the Constitution," said Mr Adam Wagner, a human rights lawyer at Soughty Street Chambers. "That is the tension."

NYTIMES