Green Party leader Zack Polanski has announced he will stand in the Holborn and St Pancras by-election, called after former prime minister Sir Keir Starmer resigned as MP. Starmer, who was replaced as Prime Minister and Labour leader by Andy Burnham in July following a series of setbacks and poor local election results, said it was the "right time" to step down and focus on international affairs. Polanski's candidacy sets up the Greens as Labour's most likely main challenger in the seat, based on trends from the 2024 general election.

Labour has held the inner London constituency since its creation in 1983, but its vote share has fallen sharply, from 64.5% in 2019 to 48.9% in 2024, shrinking Starmer's majority from 27,763 to 11,572. At the last election, independent Andrew Feinstein came second with 18.9% and the Greens third with 10.4%, with the Conservatives, Reform and the Liberal Democrats trailing further behind; both Feinstein and the Greens had campaigned strongly on Labour's stance towards Israel's actions in Gaza, a factor seen as driving some Labour voters away.