Just under two years after winning a landslide national election, Starmer saw voters punish his government, dealing it a blow in some of its traditional strongholds in former industrial regions in central and northern England.
The main beneficiary was the populist Reform UK party of Brexit campaigner Nigel Farage, which gained more than 350 council seats in England and could form the main opposition in Scotland and Wales to the pro-independence Scottish National Party and Plaid Cymru in results later on Friday.
The once-dominant Labour and Conservative parties were losing votes to Reform, to the left-wing Green Party at the other end of the political spectrum, and to the nationalists in Scotland and Wales.
Despite the losses, Starmer's allies signalled their support for a man whose popularity ratings have sunk to among the worst for any British leader.
"I am not going to walk away," Starmer told reporters in Ealing, west London, where Labour retained control of the council.
He said voters were more concerned about the pace of change rather than his leadership.
He promised to set out the steps needed to change Britain - signalling the latest reset by a government that has struggled to translate its vision for the country to voters or tackle a cost-of-living crisis that has been compounded by conflicts in Ukraine and Iran.
But there was no denying the scale of the losses for Labour in elections for 136 councils in England, and the devolved parliaments in Scotland and Wales - the most significant test of public opinion before the next general election due in 2029.
Some Labour MPs have said if the party performs poorly in Scotland, loses power in Wales, and fails to hold many of the roughly 2500 council seats it is defending in England then Starmer will face renewed pressure to quit or at least set out a timetable for his departure.
Starmer's allies warned it was not the time to move against him, with defence minister John Healey saying the last thing voters wanted was "the potential chaos of a leadership election" and that he believed the British leader could still deliver.
Reform UK leader Farage said the results represented a "truly historic shift in British politics".
His party added 367 council seats in England in early results.
Labour lost 254 seats and the Conservative Party was down 146 seats. Most of the results - including for the Scottish and Welsh elections - are due to be declared later on Friday.
Starmer was elected in 2024 with one of the largest parliamentary majorities in modern British history by offering stability after years of political chaos.
But his time in office has been marked by policy U-turns and the appointment of Peter Mandelson as Britain's ambassador to the United States who was fired nine months into the job over his links to the late convicted US sex offender Jeffrey Epstein.
