The UK's High Court has ruled that Theresa May's administration is not allowed to trigger the country's exit from the European Union, or Brexit, without approval from parliament.
Three senior judges ruled on Thursday that "the government does not have the power under the Crown's prerogative" to start EU exit talks.
The case is considered the most important constitutional matter in a generation.
The government plans to appeal against the ruling before 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.
May has said that 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.
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Claimants argue that leaving the EU will remove rights, including free movement within the bloc, and say that cannot be done without Parliament's approval.
Underscoring the importance of the case, May put Attorney General Jeremy Wright in charge of the legal team fighting the claim. Wright 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 of the EU's treaty, which starts two years of talks before 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.
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Source: News Agencies