Britain's Supreme Court began hearing a landmark case Monday that will decide who has the power to trigger the U.K.'s exit from the European Union — the government or Parliament.

The legal battle has major constitutional implications for the balance of power between the legislature and the executive, and has inflamed Britain's already raw wound over how and whether to leave the EU.

The court's most senior justice, David Neuberger, opened the four-day hearing by condemning the "threats of serious violence and unpleasant abuse" directed at Gina Miller, one of the claimants trying to ensure Parliament gets a say.

"Threatening and abusing people because they are exercising their fundamental right to go to court undermines the rule of law," Neuberger said, banning publication of the addresses of Miller and other parties in the case.

Neuberger and 10 other justices at the country's top court must decide whether Prime Minister Theresa May's government can invoke Article 50 of the EU's key treaty, the trigger for two years of divorce talks, without the approval of lawmakers.

May plans to trigger Article 50 by the end of March, using centuries-old government powers known as royal prerogative. The powers — traditionally held by the monarch but now used by politicians — enable decisions about joining or leaving international treaties to be made without a parliamentary vote.

Financial entrepreneur Miller and another claimant, hairdresser Deir Dos Santos, went to court to argue that leaving the EU would remove some of their rights, including free movement within the bloc, and that shouldn't be done without Parliament's approval.