LONDON — The British government can't unilaterally trigger the process of leaving the European Union without approval from lawmakers the High Court ruled on Thursday, a decision that is a significant blow to Prime Minister Theresa May and could complicate the timing and terms of the United Kingdom's exit negotiations with the EU.

The government said it was "disappointed" with the ruling and that it would appeal to the Supreme Court. The decision came after legal action was brought by British businesswoman and philanthropist Gina Miller, and Deir Dos Santos, a hairdresser. The two chief claimants argued that the government does not have the authority to start the process of leaving the 28-nation bloc without parliamentary approval.

May had argued that because a majority of British voters backed an exit from the EU, known as Brexit, in a referendum on June 23 the result authorized her to use royal prerogative powers to invoke Article 50 of the Lisbon Treaty, the formal legislation that would start the withdrawal process. A royal prerogative technically allows the prime minister to make decisions without consulting parliament.

In the past these powers have been used to adopt international treaties, deploy armed forces, call elections, grant pardons and even revoke passports. May had pledged to trigger Article 50 by the end of March, a move that would give the U.K and EU two years to reach an arrangement over Britain's status outside the bloc.

While Thursday's ruling is unlikely to derail Brexit and May had already vowed to confer with legislators on any new agreement with the EU — for example, on whether the U.K. retains access to its tariff-free trade arrangement with EU member states or accepts the alliance's current rules on migration for EU nationals — it adds extra uncertainty to a political affair that has struggled with clarity from the very start.

"If you ever wanted to know how to pour gasoline on a populist fire then this is a great example," said Matthew Goodwin, a politics professor at the University of Kent.

"To many (Brexit supporters) this will look like nothing short of a great betrayal, an attempt to undermine the sovereignty of the people. There is, however, little chance that the referendum result will be changed or that Britain will delay its exit from the EU indefinitely," he said. "Members of Parliament might get a vote (but) it would be political suicide for them to ignore the wishes of their constituents."

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