The Holborn and St Pancras by-election could be “make or break” for Zack Polanski if he is selected as the Green Party’s candidate, polling guru professor Sir John Curtice has warned.

He told the contest would be a “severe test” of the Green Party leader’s abilities, warning it is likely to be a “tough gig” for the party given how successful Labour has been in the seat historically.

It comes after Mr Polanski, who is currently not an MP but sits on the London Assembly, said he wants to stand in the contest, triggered by the resignation of former prime minister Sir Keir Starmer.

The Greens are expected to be Labour’s main challenger at the forthcoming race if trends seen at the last general election are repeated.

Prof Curtice said Mr Polanski has "got a chance” of winning the by-election, but said it is "probably not a high chance”.

“It's a bit like Makerfield. Burnham was taking on a set where, a few weeks earlier, Labour had lost spectacularly in the locals. But given the Greens success in the locals [elections], Polanski here would be taking on less of a challenge than Burnham did [in Makerfield]”, he said.

“In the same way that Makerfield was a test of Burnham’s abilities, it would be a severe test of Polanski’s… it could be make or break for him because not everybody is entirely happy with the way he's pushed the party.

“It would be a tough gig, but of course we need to wait and see who Labour will put up.”

While Labour has held the inner London constituency continuously since the seat was created in 1983, winning at least 40 per cent of the vote at every election, Sir Keir won a much larger share of the vote at the 2019 election (64.5 per cent) than in 2024 (48.9 per cent), with his majority falling from 27,763 to 11,572.

In 2024, independent candidate Andrew Feinstein came in second place on 18.9 per cent and the Greens in third place on 10.4 per cent.

Prof Curtice said Labour will begin the contest as the favourites to win, but they could see some of their vote share chipped away as a result of opposition to the party’s stance on Gaza.

While he said the Greens did “reasonably well” in Camden in the local elections, he warned: “Two things have then happened since then. One is that support for the Greens has fallen away a bit further than it already had done by then.

"Polanski's personal popularity has taken a bit of a hit as well. He hasn't really quite recovered from the tweet about the treatment on of the alleged perpetrator on Golders Green, so it doesn't look as good for the party.

"Secondly, Burnham has been trying to cauterise what we might call the 'Gaza wound' by saying he's going to put sanctions on people involved in the West Bank.”

Mr Polanski faced backlash in May after he reposted a tweet criticising police officers for kicking the head of the Golders Green stabbing suspect in order to get him to relinquish his knife. The row came after two Jewish men, Shloime Rand and Moshe Shine, were stabbed in London.

The popularity of the independent and Green candidates at the 2024 contest in Holborn and St Pancras is expected to have been partly down to former Labour voters protesting at Sir Keir’s refusal to take a stronger position on Israel’s actions in Gaza in the war with Hamas.

But one of Mr Burnham’s first acts since becoming an MP was to issue a video on social media in which he dramatically apologised for Labour’s response to the conflict in Gaza as he sought to win back voters who had abandoned the party over its position on the conflict.

Announcing his plan to contest the seat, Mr Polanski told the Camden New Journal: “I have lived and worked in this community over many years; I know it well and I love it.

“It represents so many of the things that make London so special. I am so excited by the prospect of representing this place in parliament.”

Mr Polanski said the constituency had “been held back by governments who have failed to make the changes to improve people’s lives”.

He added: “We know that the Greens can win here, and by electing me, voters will have someone who will always fight their corner – never scared of big donors or the party whip.

“Too often Labour has stood up for corporations and big money. I will stand up for people struggling to pay their bills.”

Mr Polanski is yet to have been formally selected by local party members as their candidate for the race, but his comments suggest he is confident this will happen.

Reports in recent days have suggested there are splits among Green Party activists about whether they should instead choose an explicitly pro-Gaza candidate for the race.

Sir Keir announced his plan to quit parliament earlier this week, saying he felt it was the “right time” to step down as an MP and focus on new work on international affairs.