Keir Starmer will be Britain's next prime minister after centre-left Labour Party won the general election, ending 14 years of right-wing Conservative rule.

Labour crossed the 326-seat threshold for a working majority in the 650-seat parliament. It is forecast to win a landslide with 410 seats.

"A mandate like this comes with a great responsibility," Starmer told supporters at a triumphant dawn rally in London, moments after the results that sealed its landslide win were announced.

The 61-year-old pledged to start work straightaway on improving the country.

"Change begins now. And it feels good, I have to be honest. Four-and-a-half years of work changing the party. This is what it is for, a changed Labour Party, ready to serve our country. Ready to restore Britain to the service of working people," Starmer said in his statement.

Sunak will tender his resignation to head of state King Charles III, with the monarch then asking Starmer, as the leader of the largest party in parliament, to form a government.