Zack Polanski, the Green party leader, has been selected as its candidate for the Holborn and St Pancras byelection on 8 October.

The seat became vacant this month when the former prime minister Keir Starmer announced he was leaving parliament.

Polanski has said the Greens, which made gains in the area in the local elections in May, are underdogs in the race but he is confident they can beat Labour, which won the seat with a majority of more than 11,000 at the 2024 general election.

On Wednesday, Labour selected the Camden council leader, Sagal Abdi-Wali, as its candidate. She said: “This place welcomed my family as refugees and gave us hope when we needed it most. I am so grateful to have had the opportunity to build a life here, serving our community as a councillor, school governor and community activist.”

Some in Labour are understood to be concerned about the impact of upcoming oil and gas announcements on the campaign. Industry insiders expect the government to greenlight the Jackdaw gasfield as soon as next week, but there are expectations that any announcement on Rosebank, a contentious oilfield, will be delayed until after the byelection.

Polanski won the race to stand for the Greens over one of the party’s local councillors, Hamza Chowdhury, who had said the constituency did not need “another MP who arrives during an election campaign” and “disappears once the votes are counted”.

At a meeting to announce his candidacy, Polanski said: “The Greens might be the underdog in this election, but we have the momentum, and we can win.

“And it’s clear everyone knows it’s the Green party against Labour, and before May we had one councillor here, and now we have 10. And last weekend we had so many people on the streets that we delivered leaflets to the entire constituency in just a few hours.”

Announcing him as the candidate, Hannah Spencer, the MP for Gorton and Denton, acknowledged that the Labour party has the advantage. She said, “the next few weeks will be tough, of course. Labour have donations from fossil fuel lobbyists and gambling industries, and they have a big machine. But we are powered by people who care, and we have shown that we can beat the establishment parties, the billionaires, and I am living proof of that.”

Polanski said he lived in the constituency when he moved to London 20 years ago and that his barber is still there.