Britain's plans to leave the European Union were thrown into confusion Thursday after a court ruled that members of parliament must be given a say in the process.

The U.K. High Court ruled that lawmakers should vote on whether the government can begin the formal Brexit process by triggering Article 50 of the EU treaty.

Experts say parliament is unlikely to block Brexit outright. But the ruling could mean Brexit is delayed, particularly by opposition in the upper chamber -- the House of Lords. Lawmakers may get a chance to influence what kind of deal the government negotiates with the EU.

The main opposition Labour Party has already said it won't try to block Brexit and instead will use the ruling to push for a Brexit "that works for Britain, putting jobs, living standards and the economy first."

The ruling is a defeat for the government. Prime Minister Theresa May wanted to trigger Article 50 by the end of March 2017, meaning the U.K. would most likely leave the EU two years later.

The government said it was disappointed by the judgment and would appeal.

"The country voted to leave the European Union in a referendum approved by Act of Parliament. And the Government is determined to respect the result of the referendum," a government spokesperson said.

Government lawyers had argued that May could give the EU formal notice of Britain's decision to leave without consulting parliament. They said the people who brought the case -- investment manager Gina Miller and hairdresser Deir dos Santos -- were trying to overturn the result of the Brexit referendum in June, in which 52% voted in favor of leaving the EU.

Three of Britain's most senior judges disagreed.

"We decide that the government does not have power ... to give notice pursuant to Article 50 for the U.K. to withdraw from the European Union," they said, according to a summary of the judgment.

Britain's Supreme Court is likely to hear the appeal in early December.

Fears that the government may pursue a "hard" break with the U.K.'s biggest trading partner have trashed the pound since the June 23 referendum.

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The currency rallied more than 1% after the ruling was announced, but has still lost nearly 17% against the U.S. dollar since the vote.

Brexit campaigners reacted angrily.

Nigel Farage, leader of the UK Independence Party, said he feared voters would be betrayed.

"I now fear that every attempt will be made to block or delay the triggering of Article 50," he said. "If this is so, they have no idea of the level of public anger they will provoke."

But Miller said she brought the case to uphold the "highest standards of transparency and democratic accountability."

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"It's not about how anyone voted," she said. "Everyone of us voted for the best country and the best future. This case was about process, not politics."