LONDON -- Britain's plans to leave the European Union hit a large speed bump Thursday, as the High Court ruled that the government can't start exit negotiations without a vote in Parliament.

The judgment deepened Britain's divide over Europe, raising hopes among pro-EU politicians that they can soften the terms of the U.K.'s withdrawal from the bloc. "Leave" campaigners say any attempt to do that would be a betrayal of voters' decision.

The government immediately said it would ask the Supreme Court to overturn the ruling. The Court has set aside time in early December to hear the case.

Thursday's ruling could delay government plans to start talks on Britain's EU exit, or Brexit, within weeks, and opens a major constitutional battle over the balance of power between Parliament and the government.

Brexit Secretary David Davis said Britain's June 23 vote to leave the EU "must be respected."

"The people want us to get on with it, and that is what we are going to do," he said.

Prime Minister Theresa May has said she will use centuries-old powers known as royal prerogative to invoke Article 50 of the EU treaty, which launches two years of exit negotiations, by the end of March.

The powers -- traditionally held by the monarch but now used by politicians -- enable decisions about international treaties and other issues to be made without a vote of Parliament.

Several claimants, including a hairdresser and a financial entrepreneur, challenged May's right to act. They argued that leaving the EU will remove rights, including free movement within the bloc, and that it couldn't be done without Parliament's approval.

Three senior judges agreed, ruling that "the government does not have the power under the Crown's prerogative to give notice pursuant to Article 50 for the U.K. to withdraw from the European Union."

The judges backed the claimants' argument that the government could not remove Britons' legal rights "unless Parliament had conferred upon the Crown authority to do so."

The ruling infuriated pro-Brexit campaigners, who see the lawsuit as an attempt to block or delay Britain's EU exit.

U.K. Independence Party leader Nigel Farage, who helped lead the campaign to leave the EU, tweeted: "I worry that a betrayal may be near at hand."

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

It's unlikely the ruling will stop Britain leaving the EU eventually. Most lawmakers accept that voters' decision must be respected -- but they differ widely on what form Brexit should take and how close a relationship Britain should keep with the EU.

A majority of members of Parliament backed the "remain" side in the referendum, but could be willing to support the start of exit talks if it's clear that the government won't seek a "hard Brexit," in which Britain leaves the EU's single market.

Pro-EU legislators hope the ruling will force the government to set out its plans for exit negotiations before triggering Article 50, something May has previously ruled out.

"Of course there is a mandate for leaving the EU, and we have to accept and respect the result of the referendum," the opposition Labour Party's Brexit spokesman, Keir Starmer, told the BBC. "But the terms, and how we leave the EU, are vitally important."

Financial entrepreneur Gina Miller, a lead claimant in the case, said the lawsuit wasn't an attempt to stop Brexit -- just to ensure that Parliament is sovereign.

"I hope the MPs (members of Parliament) will do their job and debate this in a sober, grown up way," she said.

David Greene, lawyer for hairdresser Deir Santos, another claimant, said "democracy has been reaffirmed and now very much needs to show it is alive and kicking."

The pound, which has lost about a fifth of its value since the June 23 decision to leave the EU, shot back up on the verdict, rising about 1.5 per cent to $1.2493. The ruling boosted the hopes of the financial sector, which is largely opposed to Brexit, that its effects may be mitigated by Parliament.

The case is considered the most important constitutional matter in a generation, pitting the rights of Parliament against those of the executive.

Nick Barber, associate professor of constitutional law at Oxford University, said the court had ruled decisively that "you can't use executive power to overturn statutory rights."

Jeff King, professor of law at University College London, said the government's Supreme Court appeal "would be unlikely to succeed under the circumstances."

He said the High Court judges "gave a comprehensive ruling on many points, many of which could cause the government's argument to fail."

If the government loses, it will be forced to let the House of Commons and the House of Lords have a vote. It's unclear whether that would be done with a simple motion to trigger Article 50, or would need a full Act of Parliament.

Passing legislation can take months of debate, argument and amendment, so that might see the government's timetable for Brexit slipping even further.

There is a chance the Supreme Court could refer the case to the European Court of Justice, the EU's highest court, if it thinks a legal principle needs clarification.

That would be an ironic outcome given Britain's vote to leave the EU, but Barber said it's extremely unlikely.

"I think all sides would agree that would be a mess," he said.