With many results still to be announced from Thursday's vote, centre-left Labour has already won more than 326 of the 650 seats in parliament, with an exit poll suggesting it would capture about 410, Reuters reported.
On a humiliating night for Sunak, the Conservatives have so far only won 70 and were predicted to suffer the worst performance in the party's long history with voters punishing them for a cost of living crisis, failing public services, and a series of scandals.
"Tonight, people here and around the country have spoken and they're ready for change, to end the politics of performance, a return to politics as public service," Starmer said after winning his seat in London.
"The change begins right here ... You have voted. It is now time for us to deliver."
Sunak conceded defeat and said he had called Starmer to congratulate him on his victory.
"Today power will change hands in a peaceful and orderly manner, with goodwill on all sides," he said after regaining his seat. "There is much to learn and reflect on and I take responsibility for the loss to the many good hardworking Conservative candidates ... I am sorry."
Despite his convincing victory, polls have suggested there is little enthusiasm for Starmer or his party, and he comes to power at a time when the country is facing a series of daunting challenges.
SD/PR