LONDON — Rival groups protested outside the High Court in London on Thursday as lawyers inside battled over whether the British government has the power to trigger the U.K.’s exit from the European Union without approval from Parliament.

Half a dozen protesters carrying a banner asking for the EU exit known as Brexit to start now were met by a dozen other people carrying EU flags.

The case is considered the most important constitutional matter in a generation and centers around whether Prime Minister Theresa May can start negotiating Britain’s exit from the EU without a vote in the House of Commons.

The claimants’ lawyer, David Pannick, said the case “raises an issue of fundamental constitutional importance” because it hinges on the balance between the legislative and executive branches of power — Parliament and the government.

Pannick said the executive should not be allowed to remove citizens’ rights without lawmakers’ approval — but that was exactly what would happen once the formal EU exit process began.

He said many of the rights enjoyed by lead claimant Gina Miller “will be removed … for example, Ms. Miller’s right to free movement, her right to free movement of goods, her right to freedom of services across Europe.”

May has said she will invoke Article 50 of the EU treaty — triggering two years of official exit talks — by March 31. She is under pressure from lawmakers to give them a vote first, but insists that is not necessary.

The government argues that May can use the royal prerogative — historic powers officially held by the monarch but in reality exercised by politicians — to trigger Article 50. The powers enable decisions to be made without a vote of Parliament and cover matters as grave as declaring war or as basic as issuing passports.

Attorney General Jeremy Wright, lead lawyer for the government, said May can invoke Article 50 because “the country voted to leave the EU in a referendum approved by Act of Parliament.”

“There must be no attempts to remain inside the EU, no attempts to rejoin it through the back door, and no second referendum,” he said.

The case is being brought by Miller, a financial entrepreneur. The court is also considering a related lawsuit filed by London hairdresser Deir Dos Santos.

Miller, who voted to remain in the EU, insisted the lawsuit was not an attempt to block Brexit or keep 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.”

The hearing before three judges is scheduled to last until Monday.