Departures of Foreign Secretary Boris Johnson and Brexit Secretary David Davis raises likelihood of leadership challenge.

The resignation of two cabinet ministers over Brexit inside 24 hours plunged U.K. Prime Minister Theresa May into a political crisis and increased the chances of a challenge to her leadership from inside her Conservative Party. Foreign Secretary Boris Johnson became the second prominent supporter of Britain’s departure from the European Union to quit her government since Friday, when her cabinet agreed on a plan to have the U.K. adhere to many EU rules after Brexit. His resignation followed that of David Davis, the minister in charge of Britain’s negotiations to leave the EU. The resignations blew a fragile truce within the government and sent the British pound lower, falling 0.2% to $1.3256 and 0.3% against the euro on Monday. The political turmoil comes days ahead of President Donald Trump’s visit to the U.K. on Thursday and less than nine months before the country is set to leave the bloc. Mr. Johnson, a figurehead for the fervently anti-EU wing of the Conservative Party, said in a resignation letter to Mrs. May said the latest Brexit plan would keep the U.K. economy too bound by EU regulations, effectively rendering Britain “a colony.”

The Brexit dream, he said, “is dying, suffocated by needless self-doubt.” Mr. Davis, in his resignation letter, said the direction of the government’s Brexit policy “will leave us in at best a weak negotiating position, and possibly an inescapable one.” A junior minister and Brexit supporter in Mr. Davis’s department, Steve Baker, also quit. Addressing Parliament on Monday, Mrs. May said the new agreement reached by warring factions of her cabinet at her country retreat, Chequers, on Friday, “is the right Brexit deal for Britain.”

Her political future now hangs in the balance. The prime minister was already in a weakened state after losing her parliamentary majority in last year’s general election.

More From The Wall Street Journal (subscription required):