The Greens were for decades a relatively marginal force in British politics, with only one member of Parliament (MP) and a modest but growing number of local councilors. However, its prospects changed in September 2025 after Zack Polanski was elected party leader on an eco-populist platform. This new leadership defined itself against Labour Prime Minister Keir Starmer, rejecting his apologia for Israel’s genocide in Gaza as well as his fiscal austerity, anti-migrant measures, and anti-protest authoritarianism.

Over the last year, the membership of the Green Party of England and Wales (there is a separate Scottish party) has surged from around 66,000 to over 230,000. This February, the Greens’ Hannah Spencer won the Gorton and Denton by-election, defeating both Labour and Nigel Farage’s Reform UK, and in May the Greens achieved their best-ever local election results. Now Polanski is running for Parliament in the Holborn and St Pancras by-election. In this contest, he aims not just to replace Starmer (who has resigned as an MP) but to consolidate the Greens’ position as a permanent left-wing opposition to the Labour Party.

Polanski himself is unusual for the extent of his political development over a short timescale. In a new book, Russell Warfield describes how Polanski began his political career by joining the Liberal Democrats in 2015, toward the end of the party’s stint enacting austerity in coalition with the Conservatives. Polanski joined the Greens in 2017 after failing to be selected as a Lib Dem by-election candidate, all the while opposing Jeremy Corbyn’s leadership of the Labour Party from the right. Elected as a London Assembly member and deputy leader of the Greens, Polanski then moved to the left and defeated the more centrist incumbents in last year’s leadership election.