LONDON — The case is considered the most important constitutional matter in a generation: can Prime Minister Theresa May start negotiating Britain’s exit from the European Union without an act of Parliament?

Financial entrepreneur Gina Miller’s lawsuit against the government seeks to answer this as well as much bigger questions about where power lies in this nation’s democracy and whether rights can be revoked without a vote of lawmakers.

May says the June 23 referendum on EU membership gave her a mandate to take Britain out of the 28-nation bloc and that discussing the details of her strategy with Parliament would weaken the government’s negotiating position. May has announced plans to invoke Article 50 of the EU treaty, starting talks on Britain’s future relationship with the single market, by the end of March.

The lawsuit, hearings on which begin Thursday at the High Court in London, could derail those plans. But Miller, who supported the campaign to stay in the EU, says the case isn’t about blocking Brexit — a British exit from the EU — or keeping Britain in the bloc.

“It’s about democracy,” she said. “To my mind, the most dangerous precedent we’d be setting is that a government can overrule Parliament and not consult it when we are making decisions about people’s rights. And that to me is a very, very dangerous place.”

Andrew Blick, an expert on the Magna Carta at King’s College London, said the case involves an argument that dates back almost 400 years to the English Civil War as to whether power ultimately rests in the executive or Parliament.

“It’s a long, long running dispute,” he said. “It goes back to the clash between the king and the representatives of the subjects. This is just another installment of it.”

Underscoring the importance of the case, May put Attorney General Jeremy Wright in charge of the legal team fighting the suit. That announcement was made with a flourish at the recent Conservative Party conference as May underscored she would “make a success” of taking the country out of the EU.

Wright argues the suit is an attempt to put a legal obstacle in the way of enacting the referendum result.

“There must be no attempts to remain inside the EU, no attempts to re-join it through the back door, and no second referendum,” he said in a statement. “We do not believe this case has legal merit. The result should be respected and the government intends to do just that.”

At issue is May’s attempt to use Royal Prerogative, historic powers officially held by the queen, to trigger Article 50. The powers, which have in reality passed to politicians, enable decisions to be made without a vote of Parliament and cover matters as grave as declaring war or as basic as issuing passports.

Historically, Royal Prerogative has also applied to foreign affairs and the negotiation of treaties.

Miller argues that Parliament should decide whether to invoke Article 50 — after a full debate and vote — because the rights of individuals in Britain are being revoked in the process. The freedom of movement, the freedom of trade and the freedom to vote in European elections are among the rights that will change when Britain leaves the EU.

“You’re taking away individual rights as a matter of national law,” said Miller, 51, who runs SCM Direct, an online investment manager, and is also a lawyer.

Politically, though, there’s growing cross-party pressure to give Parliament a say on Britain’s Brexit strategy before Article 50 is invoked. A motion debated Wednesday in the House of Commons demands that lawmakers have a chance to “properly scrutinize” the Brexit strategy before the formal exit process begins.

May faced a rebellion from members of her own party and was forced to concede that Conservative lawmakers could support the opposition Labour Party’s motion on Wednesday. But she insisted on amendments stating that the result of the EU referendum must be respected and that the government’s negotiating position should not be undermined.

Further complicating matters for lawyers on both sides of the issue is the fact that the U.K. constitution is not a single document as in other countries, such as the United States. Instead, it is a collection of laws and precedents established over centuries, leaving gray areas that need to be decided by the courts.

Regardless of how the High Court rules, the Supreme Court has cleared a place on its calendar in December to hear an appeal.

David Allen Green, a senior consultant at the law firm of Preiskel & Co., said the litigation may be “the most important constitutional case of a generation,” due to the magnitude of the EU debate and because talented constitutional lawyers are involved on both sides.

“But its main importance is because it deals squarely with many legally unexplored areas at the very heart of constitutional law,” said Green, who isn’t involved in the lawsuit.

He said those areas include “the exact scope of executive power in regard to treaties, the impact of referendums, the proper roles of Parliament and the courts in such matters, the protection or removal of rights of UK and EU citizens, and the relationship of all of these with the devolved governments in Northern Ireland and Scotland.”

“It could be mind-blowing stuff,” he said.