LONDON—U.K. Prime Minister Theresa May was battling Monday to overcome her biggest crisis since she lost her parliamentary majority in last year’s general election after Foreign Secretary Boris Johnson became the second prominent supporter of Britain’s departure from the European Union to quit her government in less than 24 hours.
Mr. Johnson’s resignation, announced by Downing Street in a brief statement, followed that of David Davis, the minister in charge of Britain’s negotiations to leave the EU.
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