Sir Keir Starmer is under renewed pressure after the Green Party won the Gorton and Denton by-election, with Labour in third place behind Reform UK in second.
The leftwing Greens comfortably won the key parliamentary by-election in south-east Manchester, in a previously safe Labour seat.
Labour’s defeat is likely to weaken the prime minister further, following a bruising start to the year and speculation over his leadership.
The Green candidate, Hannah Spencer, took the seat with 14,980 votes after a bitter and unpredictable three-way fight.
Reform had 10,578 votes and Labour took 9,364. At the 2024 general election Labour won the seat with a 13,413 majority.
The result suggests that the Greens are able to outcompete Starmer as the primary leftwing opposition to Reform in Labour heartlands.
Spencer in her victory speech said: “Instead of working for a nice life, we’re working to line the pockets of billionaires . . . We are being bled dry.”
Reform’s candidate, Matthew Goodwin, claimed the result represented “the emergence of a dangerous sectarianism in British politics”.
“What you saw was a coalition of Islamists and woke progressives that came together to dominate the constituency.”