“I’m not going to walk away and plunge the country into chaos,” Starmer said, after his ruling Labour party lost hundreds of councillors in England.
Labour was also braced for humiliating results in voting for the devolved parliaments in Scotland and Wales due to be announced later in the day.
“The results are tough, they are very tough, and there’s no sugarcoating it,” Starmer said.
“We have lost brilliant Labour representatives across the country, these are people who put so much into their communities, so much into our party.
“And that hurts, and it should hurt, and I take responsibility,” he added.
The ballot is the biggest electoral test for the beleaguered Starmer since Labour ousted the Conservatives following 14 years in power in a landslide election victory less than two years ago.
Grim results for Labour predicted by opinion polls appeared to be being borne out in some areas first to declare.
By 8:00 am (0700 GMT), Nigel Farage’s anti-immigrant Reform UK had gained over 350 seats while Labour had lost over 240 across 40 of the 136 English councils to announce results so far.
The vast majority of results will not come until later Friday.
Big losses for Labour could amplify calls for Starmer, 63, to resign or face a long-rumoured party leadership challenge.
He insisted Friday that “days like this don’t weaken my resolve to deliver the change that I promised”.
Reform UK’s Farage said the local election results had demonstrated a “truly historic shift in British politics” and they were now “the most national of all the parties. We are here to stay”.
Before polls closed on Thursday night, The Times reported that Energy Secretary and former Labour leader Ed Miliband had privately urged Starmer to set out a timetable to step down after the elections.
But Deputy Prime Minister David Lammy insisted early Friday that a change of leadership would be a mistake.
“You don’t change the pilot during the flight, you carry on… Sometimes, particularly incumbent governments, have it hard,” he told BBC radio.
He conceded there was a “lot of frustration” but “sometimes our mistakes have been heard more than our achievements”.
– Missteps –
The ballot is deciding around 5,000 local council seats, out of 16,000, across England, while in Wales and Scotland voters are electing new devolved parliaments.
Reform and the left-wing Greens, led by self-described eco-populist Zack Polanski, are benefiting as expected from widespread disillusionment with Starmer’s government.
Critics say Starmer has swerved from one policy misstep to another, and he has been embroiled in a scandal over Peter Mandelson, who was sacked as ambassador to Washington over his links to US sex offender Jeffrey Epstein.
He has also failed to fulfil his main promise of spurring economic growth, with impatient Britons still suffering a cost-of-living crisis, including from high energy prices.
The former lawyer is now one of the most unpopular prime ministers ever.
Surveys suggest Labour will lose control of the devolved Welsh government in Cardiff for the first time since Wales got its own parliament 27 years ago.
Reform or the pro-independence Plaid Cymru are expected finish as the biggest party.
Labour is also fearful of a drubbing in Scotland, where the Scottish National Party (SNP) is expected to extend its 19-year control of the devolved parliament in Edinburgh.
– Leadership rumours –
Labour also looks set for big losses in London as the Greens pick up disaffected left-wingers in urban areas with a pro-Gaza message.
Pollster John Curtice suggested Labour could lose around 1,500 of the seats it is defending.
Kemi Badenoch’s right-wing Conservatives were also bracing for the loss of traditional strongholds.
Curtice said the results confirmed “the fracturing of British politics”.
Britain’s media is full of rumours that ex-deputy prime minister Angela Rayner or Health Secretary Wes Streeting could try to oust Starmer after the results.
Neither is universally popular within Labour, however, and would need the backing of 20 percent of the party’s MPs to launch a contest.
