The next general election is not expected until 2029, but the defeat will intensify pressure on him ahead of May local elections, when the party is predicted to also perform poorly.

University of Manchester politics lecturer Louise Thompson said it showed Starmer must now “fight a war on two fronts”.

“Whereas previously he’s focused in a much more laser-like way on Reform... Labour will need to take the Green threat much more seriously,” she told AFP.

‘Values’

The Greens had never won a parliamentary by-election before, and ran a grassroots campaign that sought to mobilise the constituency’s 28 percent Muslim population.

The party, which under Polanski has embraced a full-throated left-wing agenda including higher taxes on the wealthy, is avowedly pro-Palestinian.

In a likely harbinger for upcoming elections, its campaign was less focused on environmental concerns - the party’s traditional rallying cry - and more concerned with cost-of-living pressures and other issues.

The result was a blow for Brexit champion Nigel Farage, whose Reform party has led national polls for the past year. He called it “a victory for sectarian voting”.

“Roll on the (local) elections on May 7th,” Farage added. “It will be goodbye Starmer and goodbye to the Tory party.”

Starmer has spent much of his time in office targeting Reform, in particular by toughening Labour’s immigration policies and rhetoric.

But the stance appears to be alienating elements of the party’s left-wing base and young people.

Andrea Egan, the new leftist leader of the traditionally Labour-supporting UNISON union, said the Greens had won “because Labour under Starmer has abandoned progressive values, imitating the far-right instead of taking the fight to them”.

“If the government wants to survive it urgently needs to stand up for workers and defend the fundamental values of our movement,” she added on X.

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