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.”