In a result that underscored the breakdown of Britain's two-party politics, the loss of one of Labour's safest seats in the northwestern English region of Greater Manchester piles pressure on Starmer to prove he should keep his job following weeks of political turmoil and calls for him to resign.
The Green Party candidate Hannah Spencer, on Friday, won the contest for the vacant parliamentary seat of Gorton and Denton, with Nigel Farage's far-right Reform UK party coming second, and Labour pushed into third place.
Starmer described the result as "very disappointing", arguing that while governing parties were often punished at such elections, he understood voter anger.
"I will ... fight against the extremes in politics on the right and the left, parties who want to tear our country apart," he told reporters.
He promised to work "as long as I've got breath in my body" to deliver the change he promised when he won a landslide national election in 2024.
But his words will do little to calm some in his party, who criticised an election strategy that focused on the threat from Reform, rather than targeting the Greens.
Spencer told a news conference the party would go from strength to strength before the next national election, which must take place by mid-August 2029.
Green leader Zack Polanski said he wanted to secure at least 30 seats, possibly up to 50, in Britain's 650-seat parliament.
John Curtice, Britain's most respected pollster, called the result a "seismic moment", which means the "future of British politics looks more uncertain than at any stage" since the end of World War Two.
Starmer had staked his personal authority on Labour winning the seat by blocking one of his rivals, the popular Manchester Mayor Andy Burnham, from standing, and by visiting the constituency this week, when British leaders normally avoid campaigning in local areas if they risk losing.
The defeat comes after Starmer faced the most dangerous moment of his premiership this month when some of his MPs said he should resign over his decision to appoint Labour veteran Peter Mandelson as ambassador to Washington, despite his links to the late sex offender Jeffrey Epstein.
Labour won just over half the vote in Gorton and Denton at the last general election in 2024.
On Friday, the Green Party won 40.7 per cent of the vote.
Reform came second with 28.7 per cent and Labour finished third with 25.4 per cent.
Labour MPs say Starmer is unlikely to face an immediate threat to his position, but some on Friday questioned his election strategy.
"In an election we tried to make into one of unity versus division, the electorate chose division - just not the division we were campaigning most against," said one Labour lawmaker on condition of anonymity.
Starmer could be challenged after the May elections, some said, when Labour is expected to fare badly in local and regional polls, including for the parliaments in Wales and Scotland.
Gorton and Denton electorate was once part of Labour's coalition of industrial areas in England, considered to be so impregnable it was called the Red Wall.
But the election contest was an example of how the British electorate has become more volatile, with declining loyalty and growing support for insurgent parties on the right and left.
The result takes the Green Party's total number of seats in the House of Commons to five out of 650.
The Labour government's main challenge at the next election is likely to come from Reform, which holds only a handful of seats in parliament, but has been leading in opinion polls.
However, Friday's result shows how Reform could struggle to win in some places, particularly ethnically diverse urban areas.
