Starmer's Labour Party haemorrhaged support in areas reporting results overnight into Friday, including traditional strongholds in former industrial regions of central and northern England, along with some parts of London.
The main beneficiary was the anti-immigration populist Reform UK of Brexit campaigner Nigel Farage, which gained more than 200 council seats in England, and could form the main opposition in Scotland and Wales to the pro-independence Scottish National Party and Plaid Cymru.
The elections for 136 councils in England, alongside the devolved parliaments in Scotland and Wales, represent the most significant test of public opinion before the next general election due in 2029.
MPs in the governing Labour Party said if the party performed poorly in Scotland, loses power in Wales, and failed to hold many of the roughly 2500 council seats it is defending in England then Starmer would face renewed pressure to quit or set out a timetable for his departure.
The early results showed the continued fracturing of Britain's traditional two-party system, in what analysts say represents one of the biggest transformations in British politics in the last century.
The once-dominant Labour and Conservative parties were losing votes to Reform, and at the other end of the political spectrum to the left-wing pro-environment Green Party, while nationalist parties were expected to win the elections in Scotland and Wales.
Farage said the results were "way exceeding" his expectations and represented a "historic change in British politics".
The results were "soul-destroying", said Rebecca Long-Bailey, a Labour MP for Salford.
While incumbent governments often struggle in mid-term elections, pollsters forecast that Labour could lose the most council seats in local elections since former Prime Minister John Major lost more than 2000 in 1995, when his government was mired in endless corruption scandals.
The Reform UK party added 253 council seats in England with results in more than 4200 seats still to be counted.
The Labour party lost 185 seats and the Conservative party was down 93 seats.
Most of the election results - including the seats in the Scottish and Welsh elections - are due to be declared on Friday afternoon and evening.
Starmer was elected in 2024 with one of the largest parliamentary majorities in modern British history on the premise that he would bring stability, rather than charisma, after years of political chaos.
But his time in office has been marked by numerous policy U-turns and the disastrous appointment of Peter Mandelson as Britain's ambassador to the United States who was fired nine months into job over his links to the late convicted US sex offender Jeffrey Epstein.
