Britain’s Labour Party won a landslide election victory Friday morning, sweeping the Conservative Party out of power after 14 years in an anti-incumbent revolt that heralded a new era in British politics.

The outgoing prime minister, Rishi Sunak conceded his party’s national defeat in the early morning hours, shortly before the Labour Party’s parliamentary majority was confirmed.

"The British people have delivered a sobering verdict tonight,” Mr. Sunak said from his constituency.

He said that he had called Keir Starmer, the Labour leader and incoming prime minister to congratulate him. Mr. Sunak, who won his local constituency, said he would travel to London to deliver his formal resignation to King Charles III and apologized for his party’s performance.

Mr. Starmer, appearing before a crowd of supporters, promised to "rebuild our country,” adding: "Change begins now.”

With a handful of districts left to declare, Labour had won more than 400 seats and the Conservatives were on course for no more than 130. That would be the worst defeat for the Conservatives in the nearly 200-year history of the party.