Prime Minister Theresa May has said she will launch exit negotiations with the EU by March 31.
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.