LONDON — Prime Minister Theresa May struggled Monday to keep her government from imploding after the resignations of Foreign Secretary Boris Johnson, a tousle-haired frontman for the campaign for Britain to leave the European Union, and David Davis, her once loyal “Brexit minister” in charge of negotiating the country’s divorce.

The bombshell departure of Johnson, a flamboyant politician and former London mayor, throws the spotlight on Tory leadership. Ever since the 2017 bungled election, there has been speculation over how long May would remain in the top job. That question has never been more urgent.

But until now, it was thought there was no credible candidate who might challenge May for the keys to 10 Downing Street. The Whitehall mandarins and the British political class thought that Johnson would bide his time. But on Friday, he infamously called May’s new plans for exiting the European Union a big pile of excrement and suggested that it would be difficult for hardcore Brexit-backers like himself to back her strategy. His departure hints at the possibility that he may mount a leadership challenge.

The surprise resignations expose May to pressure from restive Conservative party members outraged over what they see as the prime minister’s plan for a “soft Brexit” that keeps Britain tied to many E.U. rules and regulations after it leaves the bloc next year.

Hard-line Brexit backers — who want May to seek a clean, decisive break from Brussels — were in open revolt over her recently revealed proposals. They denounced the latest road map as a timid capitulation: “Brexit in name only” that ignores the 52 percent of voters who opted in June 2016 to leave the European bloc.

May replaced Davis on Monday morning with 44-year-old Dominic Raab, a leading pro-Brexit campaigner during the E.U. referendum who served as her housing minister.

Where May’s Brexit plans go now is an open question. Business leaders in Britain who run companies that make airplanes and automobiles are clamoring for answers and warning that May’s government is steering the ship toward the rocks.

The appointment of Raab is unlikely to settle the waters. May still faces threats from her hard-line Brexiteers, who are openly debating a no-confidence vote that could sweep her from power.

In Parliament, May paid tribute to Davis and Johnson even though “we do not agree about the best way of delivering our shared commitment to honor the result of the referendum.”

She praised Davis for setting up a new department and Johnson for his “passion” — a comment that generated much jeering in the chamber.

The European reaction was muted on Monday.

“Politicians come and go, but the problems they have created for their people remain,” European Council President Donald Tusk said Monday of Davis’s exit, just before being informed of Johnson’s resignation. He said the same sentiment extended to Johnson as well.

Tusk added: “The mess caused by Brexit is the biggest problem in the history of EU-UK relations, and it is still far from being resolved.”

The pound sterling held steady after Davis’s announcement but slid after the Johnson resignation. Markets, meanwhile, ticked up.

President Trump is scheduled to arrive Thursday for a visit that will be closely watched for any comments on Brexit and U.S. relations with the European Union.

In his letter of resignation late Sunday, Davis told May that her tactics and proposals make it “look less and less likely” that Britain would leave Europe’s single market and customs unions — two promises May has made.

Davis warned May her approach will just lead to further demands from Brussels and will give Europe control of large swaths of the British economy.

Speaking to the BBC on Monday, Davis said he had to resign because as Brexit secretary he did not support May’s strategy and so could not do his job.

Critics of Brexit found such admissions astounding and evidence of chaos and lack of leadership — two years after the referendum and eight months before Britain leaves the union.

One of the leading campaigners for leaving the European Union, the radio show personality and European parliamentarian Nigel Farage, said: “For Brexit to succeed we must get rid of this awful, duplicitous PM.”

In another sign of widespread confusion, Steve Baker, who resigned as David Davis’s deputy at the Brexit ministry, charged on Monday that they had been “blindsided” by May’s new proposals.

Davis said of May’s approach: “It seems to me we are giving too much away too easily.”

The outgoing minister suggested that May’s promise that Britain and its parliament would “take back control” from Brussels was hollow. “This is painted as returning power back to the House of Commons,” Davis said. “In practice, it is not doing so.”

For two years, chief negotiator Davis has been the white-haired, ruddy-cheeked face of Brexit. But talks in Brussels were notoriously slow, mostly because May’s government could not — and still cannot — agree on what kind of future relationship Britain wants with Europe on trade, immigration, law, tariffs and border checks and security.

Recently it was revealed that Davis had only attended four hours of talks in Brussels in 2018, going as long as three months without meeting the EU’s chief negotiator Michel Barnier.

David Lammy, a prominent member of the opposition Labour party, derided Davis as “a man who can’t take responsibility. For two years he’s been in charge of Brexit. No one in the world is as much to blame for this monumental mess as himself.”

The prime minister’s plan for a soft Brexit was pushed forward by May at a crunch cabinet meeting at her countryside manor, called Chequers, on Friday.

In that meeting, May had appeared to win over her fractious cabinet and secure approval for her plan, which was to be published as soon as this week in a lengthy White Paper that would stake out Britain’s vision for future relations with Europe.

While May’s plan for exiting the European Union has not be fully revealed to all members of her party — let alone to parliament, the business community or the public — the brief outline that was released shows she supports a middle way of compromise with Brussels, keeping Britain closely aligned with Europe on standards, “a common rule book for industrial goods and agricultural products.”