Britain’s Conservative Party members chose Ms. Truss, the hawkish foreign secretary, over Rishi Sunak, the former chancellor of the Exchequer.
LONDON — Britain’s Conservative Party announced on Monday that its members had chosen Liz Truss to replace Boris Johnson as leader, turning to a hawkish diplomat, party stalwart and free-market champion to govern a country.
Ms. Truss, 47, prevailed over Rishi Sunak, a former chancellor of the Exchequer, whose resignation in July set in motion Mr. Johnson’s messy ouster. Her victory, by a margin of 57.4 percent to 42.6 percent, was widely expected in recent weeks after she took a commanding lead in the polls.
It makes her Britain’s fourth prime minister in six years and third female leader, after Margaret Thatcher and Theresa May.
In a businesslike speech to a party gathering after her victory was announced, Ms. Truss promised a “bold plan” to lower taxes and bolster the economy, adding: “We will deliver, we will deliver and we will deliver.”
Ms. Truss, who served in Mr. Johnson’s cabinet and was not part of the Tory rebellion that led to his departure, will formally assume the prime minister’s title on Tuesday in a meeting with Queen Elizabeth II at Balmoral Castle in Scotland, where the queen is vacationing. Mr. Johnson will bid farewell to the monarch just before that, drawing a curtain, at least for now, on his career as a frontline politician.