The Conservatives are facing one of their worst local election results in 40 years, with striking Labour gains across England and Wales in key battlegrounds they need to secure victory at the general election.

The spread of the Conservative losses led one former minister to claim there was “no such thing really as a safe Tory seat any more”, but the prime minister appeared committed to clinging on until polling day, with rebels in his own party lacking the support to oust him.

The polling expert Prof John Curtice of Strathclyde University said the results added up to “one of the worst, if not the worst” performances by the Conservatives in four decades.

The party is expected to lose up to 500 seats when all votes are counted, with Labour advancing in areas of both the “red wall” north won by the Tories under Boris Johnson and the traditional southern Conservative heartlands.

Keir Starmer hailed “seismic” results, including winning a landslide byelection in Blackpool South, with the third largest swing since the second world war, as well as mayoralties in the East Midlands, North East and North Yorkshire, which covers Sunak’s own constituency.

Labour also ousted a number of Tory police and crime commissioners, and took control of at least seven new councils, including in Buckinghamshire, Essex, Hampshire and Sussex in the south of England.

As of 10.45pm, Starmer’s party had won more than 200 new council seats to reach a total of more than 1,000 seats, while the Liberal Democrats added 90 seats to hit 500, passing the Conservatives who lost more than 370 seats to finish on 468.

But in a warning sign for the main parties, there was also a strong showing for the Green party, which won more than 150 seats and narrowly missed out on overall control in Bristol, and independent party candidates, who won 260 seats amid disillusionment with Westminster politics and Labour’s stance on Gaza.

George Galloway’s Workers’ party of Britain won four seats, while residents’ associations took 48 and the Women’s Equality party gained its first borough councillor.

A projected national share of the vote produced by the BBC on Friday suggested Labour was on 34% of the vote, the Conservatives on 25%, the Lib Dems on 17% and others on 24%.

Despite heavy losses for the government, the Conservatives pointed to pockets of success such as the Tory mayor Ben Houchen holding on in Tees Valley, and Andy Street likely to keep his mayoralty in the West Midlands on Saturday. Both men had distanced themselves from the Conservatives’ Westminster politics.

The Conservatives also very narrowly held on to Harlow council in Essex, which had been a Labour target, and there were claims that Susan Hall, the Tory London mayoral candidate, had run Labour’s Sadiq Khan closer than predicted.

Sunak appeared alongside Houchen in Tees Valley on Friday, but said little about the wider national picture. He accused Labour of trying to “stroll back in” to Tees Valley and said he was sure that the region’s voters would stick with the Tories at a general election – despite a swing in the mayoralty suggesting the opposition would have won all parliamentary seats in the area.

Sunak’s team had been braced for the possibility of a challenge to his leadership if Houchen and Street lost. However, Tory MPs said the rebels had abandoned plans to attempt to oust him and favoured getting him to take the blame for a likely defeat in a general election this autumn.

The only voice calling for him to resign outright was David Campbell Bannerman, the former Tory and Ukip MEP, who chairs the grassroots Conservative Democratic Organisation. Bannerman described Sunak as a “terrible prime minister: not a Tory, no vision, no charisma, no campaigning ability” and called for him to “step aside now and go to California where he will excel in AI”.

Kwasi Kwarteng, the Conservative MP and former chancellor, told LBC that there was “no such thing really as a safe Tory seat any more” but he also said it was not the right time to change leader as “stability and consolidation” were needed.

The results are likely to deepen the schism within the Conservatives about whether the party should swing to the right to squeeze the Reform party, led by Richard Tice, which came within 100 votes of the Tories in Blackpool South but gained no councillors.

Andrea Jenkyns, the only Conservative MP who has publicly acknowledged sending a letter of no confidence in Sunak, instead called for a “war reshuffle” to bring back former ministers Suella Braverman, Robert Jenrick, Priti Patel and Jacob Rees-Mogg. She also told GB News that Sunak could consider trying to do a deal with the Reform party, which polled 16.9% to the Conservatives’ 17.5% in Blackpool South.

But a leading one nation Conservative MP and former minister said this would be a “daft idea” that would lead Sunak to cede even more centre ground to Labour, and he would need to “hold his nerve” and hope that Reform loses its appeal at a general election.

Tim Bale, professor of politics at Queen Mary University of London, said: “The Conservatives are much more vulnerable to their left and their centre than to their right.

“It’s easy to see Reform pose a threat to some Conservative MPs but the closer they move to Reform-style policies, the more people they put off on their other flank.”

He also said Labour “perhaps weren’t doing as well as in the run-up to 1997, but if you look at where they have won, they do seem to be winning in the right places – in Swindon, Milton Keynes, Thurrock, and the East Midlands mayoralty and the big swing against Ben Houchen in Tees Valley – then a lot of parliamentary seats, if they are anything like the same way as in the locals, will end up going to Labour.”

• This article was amended on 5 May 2024 because the Women’s Equality party gained its first borough councillor, not its first ever councillor as an earlier version implied.