Labour has won the Holborn and St Pancras byelection in a high-stakes battle for the left of British politics.

The byelection, called after former prime minister Keir Starmer resigned from the seat in September, was considered a two-horse race between the Labour candidate, Sagal Abdi-Wali, and the Green party leader, Zack Polanski. Abdi-Wali received 12,292 votes, 45% of the vote, to Polanski's 8,928, 33% of the vote.

Turnout was 38.7%, the lowest for any byelection held in England during this parliament and 15.4% down from 2024.

In an emotional victory speech, Abdi-Wali, a former child refugee who came to the UK from Somalia in the 1990s, hit out at "those who seek to divide us" in what appeared to be a response to Reform UK's Zia Yusuf, who suggested she was not British shortly after the byelection was called.

"I hope this moment says something about who gets the belong. For people who look like me. I hope they see themselves reflected in this place and know that their path too belongs here. This is Camden and this is Britain," she said.

Labour's victory will be a relief for Andy Burnham in his first big electoral test as prime minister, and reinforces polling that shows he has recovered some public support lost by Starmer, his predecessor in Downing Street.

However, the low turnout suggests Burnham still faces an uphill battle in urban areas, particularly in the capital where the Greens took control of three councils in May's local elections.

Polanski's loss caps a bruising week for his eco-populist party, which fell into turmoil after it voted on Sunday to adopt a policy declaring Zionism a form of racism, raising questions about his leadership.

Shortly after polls closed, Caroline Lucas refused to say whether Polanski should stand down. Speaking to BBC Newsnight, the former Greens leader said: "I want him to show the leadership that was frankly missing on Sunday," when the motion was voted on. Polanski abstained and campaigned in the constituency instead.

The motion said Zionism – the movement for a Jewish state – should be "treated as any other form of racism" and called for Israel to be replaced by a single Palestinian state. The head of the Jewish Greens condemned the motion and said she may leave the party, although it was supported by some Jewish Greens.

The motion passed in a vote by Green members at the party conference despite opposition from four of its five MPs.

Speaking to the BBC after the result came in, Polanski insisted he would remain in his post. "Of course I can carry on as leader of the Green party," he said. "We're a party of 220,000 members. I've taken a loss tonight but our movement is so much bigger than one election or one motion and we're going to grow."

Asked after the result whether he believed Zionism was racism, Polanski said he "grew up in a Zionist household" with the "idea that region was a land without a people and a people without a land". But he said after educating himself he doesn't "believe that's true, and what we're seeing is an ongoing genocide and the displacement of the Palestinian people. I believe that's clearly racist."

Speaking to the Guardian after her win, Abdi-Wali said local people at the school gates found the Greens motion "divisive", adding: "It just speaks to the larger divisive discourse that set off and I don't want to engage in that kind of thing."

On Labour's stance on Palestine, she said: "The work Ed Miliband has been doing on Gaza, Palestine and Israel has resonated really well with local people. He's been taking action and not just speaking."

The Greens' standing in the polls, and Polanski's personal approval ratings, had already fallen in recent months, after a peak in popularity last summer, exemplified by the Greens' seismic win in the Gorton and Denton byelection in February.

The Greens ran a strong ground game, dispatching thousands of members to knock on doors in the constituency, even before Polanski was selected as the party's candidate.

Liam Conlon, Labour MP for Beckenham and Penge and the campaign leader for Abdi-Wali said of the low-turnout: "We performed better tonight than we did in May at the local elections here. That shows you the direction of travel."

Midway into the race, George Galloway, running in "self-exile" out of outside the UK, put his name on the ballot in a move that many Green supporters said could pull votes away from Polanski. He received 314 votes.