LONDON (AP) —  In a major blow for Britain’s government, the High Court says the prime minister can’t trigger the U.K.’s exit from the European Union without approval from Parliament.

The government is likely to appeal Thursday’s ruling to the Supreme Court.

Plans for Brexit are being challenged in a case with major constitutional implications, hinging on the balance of power between Parliament and the government.

Prime Minister Theresa May has said she will launch exit negotiations with the EU by March 31. She is relying on a power called the royal prerogative that lets the government withdraw from international treaties.

Claimants argue that leaving he EU will remove rights, including free movement within the bloc, and say that cannot be done without Parliament’s approval.

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

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.

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