LONDON — For hundreds of years, the royal prerogative has allowed Britain’s leaders to mint coinage, requisition ships, send troops into battle or authorize the mining of precious metals.
But should a set of archaic rules also be used to take Britain out of the European Union?
At the Royal Courts of Justice in central London, that question was being tested last week at the start of a legal case that could — just possibly — keep Britain inside the 28-nation bloc, at least pending a more comprehensive plan for withdrawal, known as Brexit.
To date, the Conservative government has dismissed the case as legal “camouflage,” a thinly disguised effort to frustrate the will of the people expressed in a June plebiscite.
Dominic Raab, a Conservative lawmaker and Brexit supporter, told the BBC that the challenge was designed to “steal the referendum by the back door.”
But not so fast, experts in British constitutional law say.
“I think it’s enormously important,” said Jeff King, a professor of the law at University College, London, “probably one of the most significant — if not the most significant — cases of its kind of the century, and the 20th century.”
“Rarely has the legality of the execution of prerogative power had such political significance,” added Professor King, a Canadian national who sympathizes with the legal arguments of the challengers.
At issue is whether Prime Minister Theresa May is entitled to use the age-old powers of royal prerogative to start the two-year process of Britain’s disengagement from the European Union, thus sidestepping a parliamentary vote.
Mrs. May believes that the referendum vote in June to quit the bloc gives her the clear democratic mandate to start Brexit talks (she says no later than the end of March).
In doing so, she argues, Britain will sever an international treaty, which is a decision properly for the executive, not Parliament.
But that attempt to bypass Parliament may be taking Mrs. May into a legal, political and constitutional quagmire.
The case illustrates the peculiarity of Britain’s unwritten constitution, which has evolved over the centuries, rather than being codified like that of the United States and most other democracies.
Though Britons tend to like the constitutional flexibility this fuzziness provides, it also leaves gaps on questions that might be clearly answered had earlier generations written things down.
And arguments about the royal prerogative are nothing new.
In the 17th century, they formed the basis of bitter and violent disputes between Parliament and the sovereign, culminating in the English Civil War and, in 1688, the Glorious Revolution.
“The conflict was resolved only after the execution of one king and the expulsion of another,” says a research note from the House of Commons library, referring to the beheading of Charles I in 1649 and the ouster of James II in 1688.
Nowadays, such battles take place in court, and the current challenge is being mounted by several individuals, including Gina Miller, an investment fund manager, and Deir Dos Santos, a hairdresser.
They are challenging royal prerogative powers that, in modern times, are exercised by the government, not the monarch.
In this case, Mrs. May argues that, because it covers international treaty-making, the royal prerogative can be used to activate Article 50 of the European Union’s treaty without the approval of lawmakers.
The step is significant because, once invoked, Article 50 sets a two-year deadline for the conclusion of exit talks, after which Britain is likely to be outside the bloc.
Mrs. May’s critics argue that Parliament would be turned into a bystander in the process of starting Britain’s disengagement from more than four decades of European integration.
It would then be forced to pass legislation simply giving effect to a decision taken by executive fiat.
Even if Parliament gets the chance to vote at the end of the two-year period on any exit agreement, it may face a Hobson’s choice between endorsing an agreement it dislikes or being outside the bloc without any trade deal at all.
In a wood-paneled, chandeliered courtroom, the language was legalistic, and the case could hinge on an interpretation of what actually happened when Britain joined the predecessor of the European Union in 1973, a step that was approved by Parliament.
One question is whether lawmakers effectively legislated to join (in which case, one could argue, they need to legislate to leave).
Or did Parliament simply put into domestic British law the result of an international treaty (in which case it probably need not legislate to quit)?
In courtroom No. 4, one of Britain’s star lawyers, David Pannick, outlined the basis for the challenge, pointing out that the referendum in June was, in strictly legal terms, advisory.
Through the parliamentary act that in 1972 approved Britain’s membership in the European Economic Community (the predecessor of the European Union), Parliament had, he said, given Britons a range of rights, some of which had been augmented over the decades (such as the right to live and work on the Continent).
The government, he argued, proposed to use prerogative powers to do something that “defeats” rights given by the British Parliament.
“Our legal claim is in support of parliamentary sovereignty,” he told the court. “What Parliament does with its sovereignty is, of course, entirely a matter for Parliament.”
Given that many who campaigned for Brexit did so in the name of parliamentary sovereignty, the question is delicate among some supporters of withdrawal within Mrs. May’s ruling Conservative Party.
In recent years, the prerogative powers have been in retreat, particularly the prime minister’s right to take the country to war.
By deciding to consult Parliament in 2003 over the Iraq invasion, Tony Blair, the Labour prime minister at the time, made it almost impossible politically for his successors to avoid doing the same.
When asked, a decade later, British lawmakers prevented British participation in the bombing of Syria.
Were the government to lose the Article 50 case, it would pose a new parliamentary hurdle to Mrs. May at a moment when the pound sterling has fallen and the business community has become jittery about the prospect of losing preferential access to lucrative European markets.
But it is far from clear that even a defeat for the government would ultimately do anything more than complicate the process of Britain’s departure.
While the majority of its lawmakers, including Mrs. May, wanted to remain in the European Union, most of the Conservative Party has now united around Brexit, and some opposition Labour representatives would also be reluctant to oppose the outcome of the referendum.
Nevertheless, the legal action poses a test for Britain’s rickety constitutional arrangements, and also for the man formally named as the defendant in the case: David Davis, the secretary of state for exiting the European Union.
Mr. Davis, you see, is a fierce advocate of civil liberties, and campaigned, unsuccessfully, in 1999 for some crown prerogative powers to be curbed.