Theresa May Is Britain’s New Prime Minister After David Cameron’s Resignation

After a startlingly swift transfer of power, Theresa May became Britain’s prime minister on Wednesday, promising to honor last month’s referendum to leave the European Union, but also to heal divisions that the vote exposed, and to preside over an economy that benefits everyone, not just the “privileged few.”

In a surprise appointment, Ms. May named Boris Johnson, the former London mayor who became a fiery advocate of the so-called Brexit campaign only to abruptly pull out of the race to become the next prime minister, as foreign secretary. That gave the key foreign policy role to perhaps the highest-profile supporter of withdrawal from the European Union as the government prepares to carry out that process.

Speaking as she arrived at the prime minister’s office at 10 Downing Street, Ms. May, who had served for six years as home secretary, sought to position herself firmly in the tradition of “one nation” Conservatism, stressing her commitment to helping the underprivileged and pledging to fight “burning injustice.”

“We are living through an important moment in our country’s history; following the referendum, we face a time of great national change,” Ms. May said, as her husband, Philip, stood nearby.

Ms. May had supported Britain’s remaining in the European Union, but tepidly, and she promised to respect the outcome of the June 23 vote. “As we leave the European Union we will forge a bold new positive role for ourselves in the world, and we will make Britain a country that works not just for a privileged few but for every one of us,” she said.

After the fast-paced events of recent weeks, a day of political ritual saw David Cameron address lawmakers for the last time as prime minister, before tendering his resignation to Queen Elizabeth II, to make way for Ms. May, 59, who takes over at a time of flux and uncertainty not seen in decades.

Only on Monday did Ms. May learn that she would become prime minister, when the last remaining contender to lead the ruling Conservative Party, Andrea Leadsom, quit the race.

Ms. May is the 13th prime minister to serve this queen — the first was Winston Churchill — and her task is more formidable than that of most of her predecessors.

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