Standing outside his new office and residence at No.10 Downing Street, Starmer acknowledged the scale of the challenges that faced him after his party's landslide victory in a parliamentary election ended 14 years of often-tumultuous Conservative government.
He was greeted by huge cheers on Friday and shook hands with and hugged aides and well-wishers who lined Downing Street.
Prime Minister Keir Starmer and his wife Victoria greeted supporters outside 10 Downing Street. (AP PHOTO)
Standing behind a lectern, he said he understood that many Britons were disillusioned with politics after years of scandal and chaos under the Conservatives, who were roundly rejected in Thursday's election, suffering a historic loss.
"This lack of trust can only be healed by actions, not words. I know that," he said.
"Whether you voted Labour or not, in fact, especially if you did not, I say to you directly: my government will serve you. Politics can be a force for good. We will show that."
The centre-left Labour won a massive majority in the 650-seat parliament, prompting Rishi Sunak's resignation on Friday morning.
Starmer then went to meet King Charles at Buckingham Palace and was formally named prime minister.
"My government will fight every day until you believe again. From now on, you have a government unburdened by doctrine, guided only by the determination to serve your interest," he said, underlining something he had repeated during the campaign - that he would put country first, party second.
"To defy, quietly, those who have written our country off. You have given us a clear mandate, and we will use it to deliver change."
Keir Starmer became prime minister after meeting King Charles. (AP PHOTO)
Labour won some 410 seats, an increase of 210, while the Conservatives lost about 250 MPs, including a record number of senior ministers and former prime minister Liz Truss.
Sunak's Conservatives suffered the worst performance in the party's long history as voters punished them for a cost-of-living crisis, failing public services and a series of scandals.
"To the country I would like to say first and foremost I am sorry," Sunak said in a final speech outside Downing Street, adding he would stay as Conservative leader until the party appointed his replacement.
"I have given this job my all, but you have sent a clear signal that the government of the United Kingdom must change, and yours is the only judgment that matters.
"I have heard your anger, your disappointment and I take responsibility for this loss."
Despite Starmer's convincing victory, polls have suggested there is little enthusiasm for Starmer or his party.
Labour's triumph was achieved with fewer votes than it secured in 2017 and 2019 - the latter its worst result for 84 years.
Rishi Sunak apologised and took responsibility for the Conservatives' defeat. (AP PHOTO)
Britain's tax burden is set to hit its highest since just after World War II, net debt is almost equivalent to annual economic output, living standards have fallen, and public services are creaking, especially health care.
"I don't promise you it will be easy," Starmer said, adding that changing a country was "patient, determined, work".
Some of Labour's more ambitious plans have already been scaled back, while Starmer has promised not to raise taxes for "working people".
Likewise, he has promised to scrap the Conservative's controversial policy of sending asylum seekers to Rwanda, but he will be under pressure to find a way to stop tens of thousands of people arriving across the Channel from France on small boats.
Britain's election result showed growth in support for the right-wing Reform party, led by Nigel Farage, echoing recent similar results in Europe where the far right have been surging.
Starmer promises to improve relations with the European Union to resolve issues created by Britain's split from the bloc, but despite opposing Brexit, he says rejoining the EU is not on the table.
In foreign policy, Starmer has vowed to continue London's unequivocal support for Ukraine in its war against Russia.
The election victory represents an incredible turnaround for Starmer and Labour, which critics and supporters said was facing an existential crisis just three years ago when it appeared to have lost its way after its 2019 drubbing.
