Keir Starmer is all but certain to become the next prime minister of Britain, after an exit poll projected that his Labour Party would win a parliamentary elections in a landslide on Thursday.

The exit poll, which accurately predicted the winner of the last five British general elections, indicated late Thursday that Labour was on course to win a commanding majority of seats in the British House of Commons. That would mean Mr. Starmer would replace Prime Minister Rishi Sunak, who took office less than two years ago.

Mr. Starmer, a 61-year-old former human rights lawyer, has led a remarkable turnaround for the Labour Party, which just a few years ago suffered its worst election defeat since the 1930s. He has pulled the party to the political center while capitalizing on the failings of three Conservative prime ministers.

“He has been ferociously — some would say tediously — boring in his discipline,” Jill Rutter, a research fellow at the London research group U.K. in a Changing Europe, told The New York Times recently. “He’s not going to set hearts racing, but he does look relatively prime-ministerial.”

Mr. Starmer was raised in a left-wing, working-class family in Surrey, outside London. He was not close with his father; his mother, a nurse, suffered a debilitating illness that took her in and out of the hospital. Mr. Starmer became the first college graduate in his family, studying first at Leeds University, and then law at Oxford.

He was named after Keir Hardie, a Scottish trade unionist who was Labour’s first leader. As a young lawyer, he represented protesters accused of libel by the fast-food chain McDonald’s, and he later rose to become Britain’s chief prosecutor and was awarded a knighthood.

Elected to Parliament in 2015, he succeeded the left-wing Jeremy Corbyn as Labour leader in 2020 and began remaking the party. He dropped Mr. Corbyn’s proposal to nationalize Britain’s energy companies and promised not to raise taxes on working families. He also committed to supporting Britain’s military, hoping to banish an anti-patriotic label that clung to Labour during the Corbyn era.

Mr. Starmer also rooted out the antisemitism that had contaminated the party’s ranks under Mr. Corbyn. Though he has not drawn a link between that and his personal life, his wife, Victoria Starmer, comes from a Jewish family in London.