Keir Starmer's future in the balance after Gorton and Denton by-election humiliation

He now faces intense pressure to shift his party to the left or resign.

When asked if he had considered resigning, the Labour leader vowed to "keep on fighting".

"I came into politics late in life to fight for change for those people who need it," he said.

“I will keep on fighting for those people for as long as I’ve got breath in my body.”

Hannah Spencer, a councillor and plumber, emerged victorious for the Greens in Thursday’s by-election with 14,980 votes – a majority of 4,402.

Labour MPs, unions and campaign groups have demanded a change of course from Sir Keir ahead of crucial elections in May, with warnings his days in No10 could be numbered unless the party’s fortunes improve.

What did ex-Labour deputy leader Angela Rayner say about the result?

Former Labour deputy leader Angela Rayner said the result should be a “wake-up call” for the party.

"This result must be a wake up call. It’s time to really listen - and to reflect," Ms Rayner said.

"Voters want the change that we promised - and they voted for.

"If we want to unrig the system, if we want to make the change we were sent into Government to make, we have to be braver.

"A Labour agenda that puts people first. That’s what all of us across our movement need to rededicate ourselves to this morning."

At a rally in Manchester celebrating his party’s victory, Green leader Mr Polanski said the result was “an existential crisis for the Labour Party”.

He said: “No longer can they try and scare people into saying they have to vote for something because they’re worried about the least-worst option.

“A vote for the Greens is a vote both to stop Reform, to stop Labour and for something hopeful and a plan.”

Conservative leader Kemi Badenoch, whose candidate came a distant fourth with just 706 votes, said the result “shows Keir Starmer’s premiership is finished”.

She added: “He is in office but not in power. If he had any integrity he would go.”

Reform UK reports alleged 'family voting' in Gorton and Denton by-election

What was already an intense political fight to secure the by-election win shows no sign of cooling, with Reform UK having gone to the police to report cases of alleged "family voting".

It came after election observer group Democracy Volunteers warned it had witnessed “concerningly high levels” of family voting – an illegal practice where two voters use one polling booth and potentially direct each other on voting.

Nigel Farage called for an investigation into breaches of ballot secrecy to be launched by the Electoral Commission, which had directed those who think an offence has been committed to go to police.

Mr Farage earlier claimed there was “sectarian voting and cheating” in the election after his party came second to the Greens, who dismissed the remarks as a bid to undermine the result.

He said: “Reform has today reported the many cases of ‘family voting’ to the electoral commission and the police.

“What was witnessed yesterday is deeply concerning and raises serious questions about the integrity of the democratic process in predominantly Muslim areas.

“If this is what was happening at polling stations, just imagine the potential for coercion with postal votes.

“If action isn’t taken now, then we will ensure it is after the next general election.”

Greater Manchester Police confirms it is reviewing a report after Reform report alleged 'family voting' 

Greater Manchester Police confirmed it was reviewing a report.

A force spokesman said: "We can confirm a report has been made.

"We are in the process of reviewing this report and will provide a further update in due course.”

A Green Party spokesman said: "This is an attempt to undermine the democratic result and is straight out of the Trump playbook.

“We’ve just won a historic by-election by a comfortable margin.

“We’ve shown the country that Greens can beat Reform, despite their big business donations.”