Theresa May Prepares to Stare Down Parliament in ‘Brexit’ Standoff

LONDON — Against a backdrop of rising political acrimony, Theresa May, the British prime minister, warned critics on Sunday not to thwart her timetable for withdrawal from the European Union, as she prepared for a standoff with lawmakers that could prompt calls for an early general election here.

Mrs. May, who wants to start the formal process of leaving the bloc by the end of March, now has a serious fight on her hands, after several months of facing relatively little challenge over her plans for British withdrawal, known as “Brexit.”

Judges on Britain’s High Court ruled last week that she could not start exit negotiations by invoking Article 50 of the European Union’s treaty without first consulting Parliament, where the government’s majority is slim.

The government is appealing the case to the Supreme Court, but if it loses, and then finds itself constrained by lawmakers, the temptation to seek an early general election may become overwhelming for Mrs. May.

For now, the government is playing down that prospect. Mrs. May insisted on Sunday that she had a mandate to pursue Britain’s exit without consulting Parliament. In a referendum in June, about 52 percent of voters elected to quit the bloc.

“The British people, the majority of the British people, voted to leave the European Union,” Mrs. May said at Heathrow Airport as she left for a trade mission to India. “The government is now getting on with that.”

After the court ruling, however, Mrs. May now knows there is a good chance that she may not be able to do so with the free hand that she wants. So far she has specified almost no detail about her objectives, arguing that she wants to keep her negotiating position as strong as possible.

At the heart of the dispute lies an ambiguity inherent in a referendum that asked voters to say whether they wanted to quit the European Union but that did not seek their views on what relationship should replace it.

Supporters of Brexit contend that opponents now want to thwart the will of the people as expressed in the referendum. Critics fear that the government has no coherent Brexit strategy, and worry that the country may lurch into a damaging economic rupture with the bloc, which voters did not endorse.

On Sunday, Gina Miller, the founder of an investment management firm who was the lead claimant in the legal case against the government, told the BBC that Mrs. May must take the decision to Parliament “because we do not live in a tin-pot dictatorship.” Ms. Miller said she had faced online death and rape threats over the case.

Nigel Farage, the interim leader of the U.K. Independence Party, which campaigned for British withdrawal, warned of protests in the streets if the decision in favor of Brexit was ignored.

“Believe you me, if the people in this country think they’re going to be cheated, they’re going to be betrayed, then we will see political anger the likes of which none of us in our lifetimes have ever witnessed in this country,” he said.

The court ruling has unleashed an ugly political discourse, with one tabloid newspaper that supported Brexit describing the judges who delivered the verdict as “Enemies of the People.”

While the government has said it defends the independence of the judiciary, it has not rushed to condemn the newspaper coverage, prompting criticism from some senior legal figures.

More worrisome for Mrs. May is the parliamentary math should she be forced to take her case for British withdrawal to lawmakers.

Last week, David Davis, the secretary of state for exiting the European Union, conceded that if the appeal to the Supreme Court failed, the government would probably have to put forward legislation to trigger Article 50. That could give opponents the possibility to amend it, and tie down its negotiating stance.

In an interview with The Sunday Mirror, the leader of the opposition Labour Party, Jeremy Corbyn, said he would push for Mrs. May to adopt his “Brexit bottom lines.”

“We are not challenging the referendum,” he said. “We are not calling for a second referendum. We’re calling for market access for British industry to Europe.”

The party’s deputy leader, Tom Watson, took a more lenient position. He told the BBC that Labour was “not going to hold this up,” and that “Article 50 will be triggered when it comes to Westminster.”

Asked about the possibility of a general election, the health secretary, Jeremy Hunt, told the BBC on Sunday that it was “the last thing the government wants” — a formulation that does not specifically exclude it happening.