LONDON (AP) — British Prime Minister Theresa May dug in her heels Monday after the resignation of two top government ministers over Brexit negotiations whipped up a storm that threatened to topple her fragile minority government

Foreign Secretary Boris Johnson quit with a resignation letter accusing May of flying "white flags" of surrender in negotiations with the European Union. He said "the Brexit dream is dying, suffocated by needless self doubt "

Johnson followed Brexit Secretary David Davis out the door as a hard-won government consensus on future trade ties with the bloc disintegrated less than three days after it was forged, and nine months before Britain is due to leave the EU.

Davis resigned late Sunday, saying May's plan to maintain close trade and regulatory ties with the EU gave "too much away, too easily."

If Davis's resignation rattled May, Johnson's shook the foundations of her government. The tousle-headed blond Johnson is one of Britain's best-known politicians, and one of the most prominent advocates for Brexit. Some euroskeptic lawmakers dream of replacing May with a staunch Brexiteer such as Johnson, a populist, polarizing figure who has never made a secret of his ambition to be prime minister.

With Britain due to leave the currently 28-nation bloc on March 29, 2019, EU officials have warned Britain repeatedly that time is running out to seal a deal spelling out the terms of the divorce and a post-split relationship.

Minutes after Johnson quit, May defended her Brexit plan to lawmakers in the House of Commons, with Johnson absent from his usual place on the Conservative front bench.

She said she and the two departed ministers "do not agree about the best way of delivering our shared commitment to honoring the result of the referendum" in which U.K. voters opted to leave the EU.

May's plan seeks to keep the U.K. and the EU in a free-trade zone for goods, and commits Britain to maintaining the same rules as the bloc for goods and agricultural products.

May said the plan would deliver frictionless trade with Europe and was the "only way to avoid a hard border" between the U.K.'s Northern Ireland and EU member Ireland. Britain and the EU agree there must be no tariffs and immigration checks along the currently invisible frontier, but working out how to achieve that has been a major stumbling block in negotiations.

Rebuffing claims that her proposals make too many concessions to the EU, May said her "smooth and orderly Brexit" would leave Britain free to make its own laws and trade deals.

The resignations came just days after May announced she had finally united her quarrelsome government behind a plan for a divorce deal with the EU.

Government unity began to fray within hours of Friday's announcement. Brexit-supporting lawmakers were angered by the proposals, saying they would keep Britain tethered to the bloc and unable to change its rules to strike new trade deals around the world. They also argued that the proposals breach several of the "red lines" the government had set out, including a commitment to leave the EU's tariff-free customs union.

In his resignation letter, Davis said the "'common rule book policy hands control of large swathes of our economy to the EU and is certainly not returning control of our laws in any real sense."

Johnson said in his letter that May's plan to keep close economic ties with the bloc means Britain is heading for a "semi Brexit" that would leave Britain with the "status of a colony" of the EU.

Britain and the EU hope to reach broad agreement by October so the national parliaments of the remaining countries can ratify a deal before Britain leaves. The timetable increasingly looks overly optimistic, and EU frustration with British division and chaos is growing.

European Council President Donald Tusk said Monday that "the mess caused by Brexit is the biggest problem in the history of EU-UK relations and it is still very far from being resolved."

Opposition Labour Party leader Jeremy Corbyn said the government was incapable of delivering Brexit.

"How can anyone have faith in the prime minister getting a good deal with 27 European Union governments when she can't even broker a deal within her own Cabinet?" he asked.

May has hung on to power longer than many expected after she lost her majority in a June 2017 snap election that she had called in hopes of strengthening her hand in Brexit talks.

The fear among May's allies is that more resignations may follow. Steve Baker, a junior Brexit minister, resigned along with Davis. May appointed staunchly pro-Brexit lawmaker Dominic Raab as the country's new Brexit secretary. She did not immediately name a replacement for Johnson.

The loss of two senior ministers and the anger among Brexit-supporting backbench lawmakers makes May's position as leader increasingly tenuous.

Davis insisted he did not want his resignation to become a rallying cry for May's ouster.

"I like Theresa May, I think she's a good prime minister," Davis said.

But other pro-Brexit lawmakers were furious at what they saw as a sell-out of the clean Brexit they desire. Euroskeptic Conservative lawmaker Peter Bone said party activists felt "betrayed" by the government plan.

Under Conservative Party rules, a confidence vote in a leader can be triggered if 48 Conservative lawmakers write a letter requesting one.

May was asked by an opposition lawmaker Monday whether she would contest a vote of confidence rather than resign.

"Nice try," she said with a touch of bravado. "But I'm getting on with delivering what the British people want."

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Associated Press writers Lorne Cook and Raf Casert in Brussels contributed to this report.