LONDON — The British government must consult Parliament before proceeding with formal negotiations over its withdrawal from the European Union, the High Court ruled on Thursday, a move that is likely to aggravate the political uncertainty surrounding the country’s exit from the bloc.
The decision is likely to be appealed to the Supreme Court of the United Kingdom, and it might ultimately be referred to the European Court of Justice, an institution opposed by those who argued for Britain to leave the bloc.
The legal action was brought in the name of several individuals, including Gina Miller, an investment fund manager, and Deir Dos Santos, a hairdresser, who challenged the right of the government to take the formal legal step that would trigger Britain’s exit from the 28-nation European Union without consulting Parliament.
That process, invoking Article 50 of the Lisbon Treaty, would set a two-year deadline for the conclusion of talks to define Britain’s relationship with the European Union after it leaves.
Prime Minister Theresa May believes she has the right to move ahead without parliamentary approval by using royal prerogative powers that, in modern times, are exercised by the government in the name of the monarch.
The royal prerogative provides a variety of powers, from ordering troops into battle to authorizing the mining of precious metals. The government believes the royal prerogative is applicable in the case of a British withdrawal because the powers cover international treaty-making.
Both the government and its legal challengers say that invoking Article 50, which Mrs. May has promised to do by the end of March, is an irrevocable step on the road to a British withdrawal, commonly known as Brexit.
Mrs. May’s critics argued in court that failing to give Parliament a voice in the matter would turn lawmakers into bystanders as Britain negotiates its disengagement from the European Union after more than four decades of membership. They also pointed out that, technically, the referendum is not legally binding.
Although Mrs. May has said that lawmakers will eventually be consulted, many fear it would take place too late to influence the shape of Britain’s new relationship with the European Union.
For example, if Parliament is given a chance to vote on an exit agreement at the end of the two-year period, lawmakers may be forced to choose between endorsing a deal they oppose or being outside the bloc without any formal relationship with the union.
The government had dismissed the case as legal “camouflage,” a thinly disguised effort to frustrate the democratic outcome of the June 23 referendum, in which 52 percent of British voters backed leaving the bloc.
Many observers believe that, were the decision to trigger Article 50 put to Parliament, lawmakers would probably approve it, even though a clear majority opposed leaving the bloc when the referendum was held.
Despite that fact, lawmakers voted overwhelmingly to hold the referendum in the first place, making it politically difficult to fail to honor its outcome.
Moreover, the Conservative Party, which was badly split over the referendum, has now largely embraced its outcome, in many cases enthusiastically.
Many supporters of the opposition Labour Party also voted to leave the European Union, which will make it hard for their lawmakers to oppose a withdrawal.
The government is worried about delays to the timetable, however, and fear that lawmakers will seek to make their support for withdrawal conditional, for example on remaining part of the union’s single market.
Mrs. May wants the freest possible hand in negotiating with other European Union nations and has said she will not offer a running commentary on the talks.
Nevertheless, a failure to consult lawmakers has exposed Mrs. May to political criticism, not least because supporters of a withdrawal argued during the referendum campaign for the need to restore sovereignty to the British Parliament.