Starmer officially became the UK's new leader on Friday after a meeting with King Charles at Buckingham Palace following the resignation of Rishi Sunak.
The centre-left Labour won a massive majority in the 650-seat parliament, with Sunak's Conservatives suffering 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.
"We did it," Starmer said in a victory speech.
"Change begins now ... We said we would end the chaos, and we will, we said we would turn the page, and we have. Today, we start the next chapter, begin the work of change, the mission of national renewal and start to rebuild our country."
Labour campaigned on promises of sweeping change following 14 years of Conservative government. (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.
The Scottish National Party imploded, losing 38 seats, ending its own decade of dominance in Scotland and leaving its dream of Scottish independence in tatters, while the Irish nationalists Sinn Fein became Northern Ireland's largest party for the first time.
The populist right-wing Reform UK party, headed by Nigel Farage, the colourful Brexit campaigner, won more than four million votes.
While it secured only four MPs, its impact by siphoning vast tracts of Conservative support will make Farage a thorn in the side of the two major parties.
A glum Sunak said he would stand down as party leader and met King Charles to formally resign.
"To the country I would like to say first and foremost I am sorry," Sunak said outside 10 Downing Street.
"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."
Outgoing Prime Minister Rishi Sunak apologised and said "I take responsibility for this loss". (AP PHOTO)
Despite Starmer's convincing victory, polls have suggested there is little enthusiasm for him or his party.
Labour's triumph looked set to be achieved with fewer votes than in 2017 and 2019 - the latter its worst electoral showing for 84 years.
Starmer also comes to power at a time when the country faces a series of daunting challenges.
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 the cherished National Health Service.
"I don't promise you it will be easy," Starmer said.
Changing a country was hard work, "and we will have to get moving immediately".
Starmer has promised to scrap the Conservatives' controversial policy of sending asylum seekers to Rwanda, but he will be under pressure to stop tens of thousands of people arriving across the Channel from France on small boats.
Recriminations began within the Conservatives, with some saying their failure stemmed from abandoning the centre ground while others argued Reform had taken voters who felt the party had deserted its roots.
Reform UK's showing will make Nigel Farage a thorn in the side of the two major parties. (AP PHOTO)
"There is a massive gap on the centre right of British politics and my job is to fill it, and that's exactly what I'm going to do," said a triumphant Farage after finally being elected to parliament at his eighth attempt.
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 affairs, Starmer has vowed to continue UK support for Ukraine in its war against Russia.
A series of Conservative scandals - most notably revelations of parties at Downing Street during COVID lockdowns - undermined then-prime minister Boris Johnson and its commanding poll lead evaporated.
Truss's disastrous six-week premiership, which followed Johnson being forced out at the end of 2022, cemented the decline, and Sunak was unable to make any dent in Labour's commanding poll lead.
Sunak observers by calling the election earlier than he needed to while trailing badly in the polls, and his campaign proved a disaster.
