Andy Burnham has more than halved the number of voters defecting from Labour to the Greens since becoming prime minister.

Almost 70% of those who voted Labour at the last election now plan to support the party again, compared with 52% in February, when Keir Starmer was in No 10. The proportion of disillusioned Labour voters switching to the Greens has dropped from 18.5% to 8%, according to the latest Opinium poll for The Observer.

Among younger voters, support for the Greens has fallen from 28.6% around the time of the local elections in May to 19.7%, while the number of those aged between 18 and 34 planning to vote Labour has risen from 23.3% to 33.5%.

The poll suggests Burnham is successfully shoring up Labour’s left flank after the announcement of sanctions on illegal West Bank settlements and a recommitment to net zero. Labour is on 27%, 3 percentage points ahead of Reform UK, while the Greens are on 11%.

The return of progressive voters to the Labour fold will significantly boost the party’s chances of retaining power and increase pressure on Burnham to call an early general election.

Analysis by the More in Common thinktank suggested that, under Starmer, there were 72 constituencies where voters switching to other parties on the left could cost Labour the seat.

It found 47 seats where projected Green gains exceeded the margin by which Labour would lose the seat, and 25 where the expected increase in support for the Liberal Democrats would be enough to topple the Labour MP.

The defence secretary, Wes Streeting, was one of the senior figures under threat. Those seats now look much more winnable for Labour.

The first big test of Burnham’s popularity among urban progressive voters will come on 8 October in the Holborn and St Pancras byelection, created by the resignation of Starmer as an MP.

The hopes of the Green party leader, Zack Polanski, winning the seat have been undermined by George Galloway’s decision to run as a candidate for the Workers party of Britain. In recent months, Polanski has also faced criticism over his social media posts and increased scrutiny of his party’s policy positions.

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A strategy paper by ThinkLabour – formerly the Labour Together thinktank – that is circulating in No 10 argues the party can sustain its electoral coalition only if it is “clear and confident about its core values”.

Last week, Louise Haigh, first secretary, told the Trades Union Congress that the announcement on West Bank sanctions would “remind the world who we are”. Burnham also insisted he was determined to meet the net zero target on greenhouse gas emissions by 2050. The expected approval of drilling at the Jackdaw gas field in the North Sea has been delayed until after the byelection.

On Tuesday, the prime minister is expected to have his first face-to-face meeting with Donald Trump at the UN general assembly in New York. Ukraine is high on the agenda but sources say that, unlike Starmer, Burnham will not “bow the knee” to the US president. The aim in No 10 is to lower the stakes of the meeting, which will be sandwiched between Trump’s speech and Burnham’s remarks.

Jonathan Ashworth, who lost his Leicester South seat to a pro-Gaza Muslim independent candidate, said the Israel-Palestine conflict had become a defining question for voters in constituencies such as his.

“The failure to call for a ceasefire early on in the conflict, the infamous LBC interview [in which Starmer suggested Israel had the “right” to withhold power and water from Gaza] and timidity on sanctions was an indefensible position for Labour. It unsurprisingly led to vast swathes of progressive-minded voters deserting Labour,” he said. “Andy Burnham and Ed Miliband are right to take the principled stance based on Labour values they took to bolster a two-state solution. I suspect it will also aid Labour in rebuilding support on its progressive flank, helping hold scores of seats.”

The Opinium poll found that voters are more than twice as likely to support than oppose the announcement about sanctions on illegal West Bank settlements, with 38% in favour and 16% against. Among those intending to vote Green, 56% back the policy.

Luke Tryl, UK director at More in Common, said: “The Gaza issue appears to play a similar role for progressive voters to the one Brexit played in unanchoring Labour voters in the ‘red wall’. The issue was not the sole driver of discontent, but became symbolic of a mood that Labour was out of step with its voters’ values and, for some in the Muslim community, reinforced a wider sense of neglect.”

But he insisted the cost of living and affordable housing were still “more salient” to these voters. “Sanctions on West Bank settlements will signal to voters that Burnham is breaking from Keir Starmer’s approach. But it will take progress on a much wider range of issues to win back Labour’s lost voters.”

Mack Mooney, 22, who is studying in Manchester, voted Green in 2024 because he was “disillusioned” with Labour but is now part of the Burnham bounce. “The Greens felt like the party that reflected my views, and a lot of my friends around me. I felt like Starmer was adhering to the right too much,” he said.

“But I would totally vote for Andy Burnham’s Labour government, as I feel they reflect Labour’s historical core values, of looking out for working people and young people. I like some of his policies around supporting small businesses, and I think he’s taken a slightly stronger position on Israel. I like that he seems willing to be ambitious and take risks.”

Deborah Mattinson, Labour’s director of strategy at the last election, said that “of course the party needs to reassure the progressive groups in its coalition, which had been looking very frayed”. But she warned: “What hasn’t changed is that you can’t win without also winning working-class voters. You can call them hero voters or the ‘just about managing’ – they are the people who are financially vulnerable and insecure.

“For example, if their fridge or washing machine were to break down tomorrow, they wouldn’t be able to replace it without taking out a loan. It’s not a choice. You have to show you’re on their side.”

Photograph by Oli Scarff /WPA Pool/Getty Images