The Green party said its membership has passed 200,000 this weekend in the wake of its victory in the Gorton and Denton byelection overturning a huge Labour majority.

The party’s membership has tripled since September last year, when it was about 68,000, after the announcement of Zack Polanski as its new leader.

The Green victory in Gorton and Denton is its first in a national byelection, forcing Labour into third place with Reform in second.

The Greens now have five MPs and are regularly matching the Lib Dems in the polls while snapping at the heels of Labour and the Conservatives.

Labour is under pressure from some of its MPs to steer more to the left to win back progressive voters from the Greens after the byelection result, where 34-year-old plumber Hannah Spencer was elected.

Polanski said: “This membership surge proves that the future of progressive politics belongs to the Greens.” He described it as a “political turning point”, with its members joining a “movement that refuses to accept managed decline, climate delay or timid politics”.

He added: “Greens are not here to be disappointed by Labour, but to replace them. We will not wait politely for change; we are building it.”

Figures released last summer suggested Labour was still the UK’s largest political party with more than 330,000 members – down from more than 500,000 in 2019. However, a report in the Times put it lower at less than 250,000 late last year – less than Reform UK.

The number of Conservative party members is not known but is thought to be about 120,000, while Reform UK says it has more than 270,000. Figures published in August suggest that Lib Dem membership was last recorded at about 80,000.

High membership numbers do not necessarily translate into electoral success, with Labour having had a peak of more than 500,000 members under Jeremy Corbyn, who lost two general elections. However, they are an indication of how much the party is enthusing its core base whose support can lead to higher levels of volunteering and activism during an election.

The Greens’ surge has caused worries among some senior Labour figures, who believe it could take votes and seats from the party in May’s local elections in England, particularly in some London boroughs.

Polanski has repeatedly set out what he sees as the Greens’ ambition to supplant Labour. In his speech to his party’s annual conference, he said that without action Keir Starmer would “hand this country on a plate” to Reform UK.