LONDON (NYTIMES) - 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 (Sept 24), 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".

But critics warned that in taking upon itself the right to adjudicate disputes between the government and Parliament, the court would soon find itself thrust into the same charged, highly partisan waters that have politicised decisions of the US Supreme Court in recent decades.

The sweeping nature of the ruling surprised legal scholars, even those who had predicted that the court would find some way to rebuke Mr Johnson for his widely protested decision to suspend, or prorogue, the Parliament earlier this month.

"It's absolutely stunning," said Dr James Grant, a senior lecturer in law at King's College London.

"It's stronger than a lot of us expected. 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, however, expressed regret that the court had felt compelled by Mr Johnson's actions to cast off a long tradition of legal restraint.

Brexit has pushed the court, as it has Parliament itself, into wholly new terrain, where old rules and traditions no longer seem to apply.

"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, but 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 the court's president, Baroness 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's far more similar to the sort of judicial review you'd get in a country with a codified Constitution. But to arrive at its conclusion, the court has had to pluck unwritten constitutional principles from what they take to be political practice."

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.

The momentous nature of the decision was evident in the live television coverage of the hearing on Tuesday morning.

Justice Hale and 10 fellow justices took their seats behind a polished semicircular table. (Britain's high court, unlike its American counterpart, allows cameras in the courtroom.)

Clad in black and wearing a large bejewelled brooch in the shape of a spider, Justice Hale read the ruling aloud in a quiet, firm voice.

She emphasised that the decision, which came after three days of legal submissions, had been unanimous.

The justices, she said, had weighed the court's role in judging the prerogative powers of the government, in cases dating back to 1611.

There was no doubt, Justice Hale said, that the court had jurisdiction, since the case involved an action that prevented Parliament from fulfilling its constitutional duty.

Lawyers for the government had argued that the courts had no jurisdiction, an argument that an English lower court had accepted.

"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.

She said it was up to the Speaker of the House of Commons to decide when it would reconvene.

The speaker, Mr John Bercow, said the House would come back into session on Wednesday morning, three weeks before the date the government had set.

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

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

Mr Sumption, the former Supreme Court justice, said that while he wished the court had not felt obliged to act, the effect of the decision was "to reinstate the democratic legitimacy of public decision-making".

Prorogation, he added, was a procedural issue rather than a substantive government policy.

For some legal experts, the decision underscored the need for Britain to confront the costs of its uncodified Constitution.

"This is constitutionality on the fly," said Mr Adam Wagner, a human rights lawyer at Doughty Street Chambers.

Although the ruling could be seen as a validation of Britain's flexible constitutional system, Mr Wagner said, it also highlighted the legal morass that encompasses so many basic decisions of the prime minister.

The ruling, he said, entrenched an emerging principle that the prime minister is accountable to Parliament.

But in so doing, it 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.

Where the courts view themselves as defending existing constitutional principles, critics have accused them as making things up.

"The courts see it as revealing the Constitution, but many people will see it as inventing the Constitution," Mr Wagner said. "That's the tension."