LONDON (AP) — Britain's High Court is set to rule Thursday on whether the government can trigger the U.K.'s exit from the European Union without approval from Parliament.

The case is considered the most important constitutional matter in a generation.

It centers on whether Prime Minister Theresa May can start negotiating Britain's exit from the EU without a vote in the House of Commons. At the core of the matter is the balance of power between Parliament and the government.

Financial entrepreneur Gina Miller, who supported the campaign to stay in the EU, brought the case. She argued that leaving the EU would deny her fundamental rights, such as the right to free movement and freedom of services across Europe.

Underscoring the importance of the case, May put Attorney General Jeremy Wright in charge of the legal team fighting the claim. Wright has argued that the lawsuit is an attempt to put a legal obstacle in the way of enacting the result of the June 23 referendum to leave the EU.

May wants to use Royal Prerogative, historic powers officially held by the queen, to trigger Article 50, which signals Britain's departure from the EU. 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.

Regardless of the victor, the case will likely be immediately appealed to the Supreme Court.

May has said she will invoke Article 50 of the EU treaty — triggering two years of official exit talks — by March 31.