Voting has opened in the Holborn and St Pancras byelection, a contest seen as a key gauge of Andy Burnham’s ability to energise voters and whether Zack Polanski’s Greens can still peel off support from Labour’s left.

Polanski, who leads the Greens in England and Wales, is standing for his party in the north London constituency, previously held by Keir Starmer before he stood down as an MP shortly after resigning as prime minister.

The seat is, under normal circumstances, a Labour stronghold. Starmer won it with a majority of 11,500 at the 2024 general election despite strong challenges from the Greens and a leftwing independent candidate, who is not standing again and has endorsed Polanski. In 2019, Starmer won by nearly 28,000 votes.

Polanski has run a highly energetic campaign. He has mobilised a large numbers of supporters to knock on doors, mimicking February’s Gorton and Denton byelection where the Greens overcame an even bigger Labour majority to take the seat.

However, the political landscape has changed. Shortly after Gorton and Denton, YouGov polling put the Greens briefly above Labour, on a peak of 21%. The most recent equivalent had Polanksi’s party on 11%, 16 points behind Labour.

Burnham, who succeeded Starmer as prime minister in July, is seen as having revitalised Labour, and used his speech at the party’s annual conference last month to set out a more ambitious and left-leaning collection of policies.

Polanski’s fortunes in contrast appear to have declined, especially since last weekend’s Green party conference where a vote by members to adopt a policy equating Zionism with racism and calling for Israel to be replaced by a single Palestinian state prompted condemnation and splits in the party.

Zack Polanski has mobilised a large number of supporters to knock on doors, mimicking February’s Gorton and Denton byelection. Photograph: Andy Hall/The Guardian

Four out of the five Green MPs have condemned the policy, while councillors and others have distanced themselves from it or in some cases left the party.

While policy is decided by Green members in direct votes, Polanski has been criticised for not saying whether or not he backed the motion and leaving the conference before the vote took place to resume campaigning.

The stakes are also high for Burnham. Failure to win the seat by a reasonable margin could end his honeymoon period and bring renewed doubts about Labour’s ability to protect inner-city seats from the Greens, who won several London councils in May’s local elections.

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Holborn and St Pancras could be promising ground for the Greens, with a young and diverse population that includes many students. About 45% of constituents were born overseas and 48% are aged between 18 and 34.

Labour has campaigned hard, fielding as its candidate Sagal Abdi-Wali, the 42-year-old leader of Camden council who arrived in London in the early 90s, speaking no English, after fleeing war in Somalia with her four brothers and her mother, who trained as a lawyer in Somalia.

In a pre-poll message to voters, Burnham said: “This area gave Sagal a home when she arrived here as a refugee in the 1990s. Camden gave Sagal and her family hope. Now she wants to give that back to the communities that have always been there for her. It’s hard to think of someone more committed to their local area and the care of people than Sagal.”

In his own statement, Polanski said the result was “looking close”, and that voters should choose him to pressure Labour.

He said: “We already have a Labour government, a Labour council and a Labour [London] mayor. Isn’t it time for a Green voice for Camden to push Labour to do better on issues like rent controls and to put Thames Water into public ownership – issues where they’ve been backtracking?”