LONDON — The British government’s plan for exiting the European Union was thrown into uncertainty on Thursday (Nov 3) after the High Court delivered a defeat to Prime Minister Theresa May by ruling that she must seek parliamentary approval before starting the process to leave the bloc.

The court’s decision seemed likely to slow — but not halt — the process for a British withdrawal from the 28-nation union, a decision approved by nearly 52 per cent of voters in a June 23 referendum.

Mrs May had insisted that the government could invoke Article 50 of the Lisbon Treaty, the mechanism for leaving the European Union, without a vote by Parliament. She immediately vowed to appeal the ruling to the Supreme Court of the United Kingdom, which is to hear the appeal in December.

Mrs May’s Conservative Party holds only a slim majority, with 329 seats in the 650-seat Parliament. Although most lawmakers opposed the decision to leave the European Union, it would be politically toxic for them to overturn the referendum outcome.

Still, the pound, which has been at a historic low against the dollar, rallied as traders apparently believe greater parliamentary involvement in the process would therefore reduce the influence of ministers in Ms May’s government who are strongly pro-Brexit.

This could reduce the likelihood of a "hard Brexit", a scenario in which Britain prioritises tight controls on immigration over remaining in the European single market.

The government and its legal challengers agree 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. Her spokesman maintained on Thursday that it was still possible to begin the process by that time.

The government is worried about delays to the timetable and fears 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.

The ruling makes the already daunting task of taking Britain out of a club it joined 43 years ago even more complex.

"The government is disappointed by the court’s judgment," Trade Minister Liam Fox told Parliament.

"The country voted to leave the EU in a referendum approved by acts of Parliament. The government is determined to respect the result of the referendum."

Making clear the government planned to stick to its timetable, the spokeswoman for Ms May said: "Our plan remains to invoke Article 50 by the end of March, we believe the legal timetable should allow for that."

The case was brought by several plaintiffs, including Ms Gina Miller, an investment fund manager, and Mr Deir Dos Santos, a hairdresser.

Ms Miller said the case was about "process, not politics" and rejected accusations of trying to usurp the will of the people.

"One of the big arguments (in the referendum) was parliamentary sovereignty," she told reporters. "So you can’t on the day you get back sovereignty decide you’re going to sidestep or throw it away."

Lawmakers largely voted to remain in the EU in the June referendum.

Mr Nigel Farage, head of the anti-EU party UKIP, said on Twitter that he feared the ruling could turn into an attempt to scupper Brexit altogether.

"I worry that a betrayal may be near at hand," he said. "I now fear every attempt will be made to block or delay triggering Article 50. They have no idea (of the) level of public anger they will provoke." AGENCIES