LONDON (AP) — Rival protesters gathered Thursday outside the High Court in London, where lawyers battled over whether the government has the power to trigger Britain'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.

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.

Financial entrepreneur Gina Miller, the lead claimant in the case, insisted the lawsuit isn't 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.