The Green Party of England and Wales has more than tripled its membership in the six months since Zack Polanski became leader, according to party figures, as the rapid influx of activists forces the party to expand its campaigning, policy operation and local organisation at speed.

Membership has risen from about 66,000 before Polanski took over last September to more than 215,000 in early March, with the total passing 100,000 in October and then 200,000 after last month’s Gorton and Denton by-election. Party officials say around 15,000 people joined in the week after that victory alone.

The rise gives the Greens far greater manpower and a much stronger small-donor base as the party tries to convert local gains into a wider parliamentary breakthrough. But organisers and officials say the scale and speed of the increase is also bringing strains, with a majority of members now having joined within the past six months.

“We are, in effect, a completely different political party,” one experienced Green organiser said. “The majority of people have been around for less than six months.”

The surge has been linked by party figures to disillusionment with Labour in government, as well as the turmoil that has affected Jeremy Corbyn’s Your Party. Several organisers said a sizeable share of the newcomers had either left Labour or turned to the Greens after becoming frustrated with Your Party’s internal problems.

Some longstanding members say the new intake has changed the tone of local parties. One organiser said some arrivals had brought a “Corbyn-ish” political culture that seemed “more concerned about winning the argument and factionalism than getting power”.

There have already been signs of friction. Party figures described local campaigning mistakes, including new members putting Palestinian flags on leaflets that were later distributed in more affluent suburban areas. Some outside observers have warned that the Greens risk a de facto takeover by activists from the organised left if the party cannot absorb the influx quickly enough.

One Labour MP, who saw the Greens finish second in their constituency at the last general election, said their local Green party now appeared to be “full of these ultra-factional Corbyn refugees” and predicted that “it won’t go well”.

The Greens’ victory in Gorton and Denton underlined the electoral upside of the boom. Hannah Spencer won the Manchester seat on 26 February with 14,980 votes, or 40.7 per cent, overturning a seat Labour had won with just over half the vote in 2024. Reform UK came second and Labour fell to third, in a result widely seen as a significant warning for Sir Keir Starmer’s party.

Green organisers said the enlarged membership meant the party could draw on unusually large numbers of canvassers and leaflet deliverers during the campaign. The result gave the Greens their first Westminster by-election win and a fifth MP.

Polanski has argued that the surge shows the Greens are becoming the main vehicle for progressive voters unhappy with Labour. He has said the increase proves “the future of progressive politics belongs to the Greens” and has repeatedly framed the party’s ambition as not merely to pressure Labour, but to replace it.

Inside the party, however, many organisers reject the suggestion that the Greens can be rapidly captured by any one faction. They point to the party’s decentralised structure, the autonomy of local parties and a policy-making process that runs through conferences and working groups rather than through the leadership alone.

“What entryism looks like for us is people attending conference and having their voices heard,” another Green organiser said.

A party official said even if new members wanted to change policy quickly, the system made that difficult. Altering core policy is not a matter of a single vote, the official said, but a process that can stretch across multiple conferences and take around 18 months. “You do get some people going: ‘What have I just joined? This isn’t like the Labour party,’” the official said.

For many local parties, the immediate challenge is less ideological than practical. Some are said to be taking in as many as 500 new members a month, creating pressure to welcome people, provide training, find roles for them and maintain message discipline across fast-growing campaigns.

“In any sudden growth there are always adjustments,” one senior Green said. “But there is a lot of extra energy. And it’s great to have friends who have never been especially political message me to say they have signed up.”

The financial impact is also significant. With membership income rising sharply, the party expects its 2026 budget to more than double compared with the previous year. That increase is expected to support a professionalisation drive, including the recruitment of new media and policy staff.

At present, Green officials say, policy groups are largely chaired and staffed by volunteers. One official said some of those groups were led by “real experts”, while others were “nightmares”, and argued that extra funding would allow national policy work to be overseen more consistently.

Some organisers say the cultural change could ultimately help the party. Before the boom, one said, local meetings could be “quite cosy”, with longstanding members spending hours on narrow policy detail. The arrival of 100 or 200 new members in some areas, they said, is forcing local parties to think more seriously about campaigning, voter appeal and how to win power.

“People are being challenged,” the organiser said. “They are having to move away from what you could call the comfort blanket of being right, or feeling like a big fish in a small pond. It’s now a much bigger pond.”

Even so, party figures say some of the Greens’ old habits remain intact. One organiser said Labour councillors who had defected to the party had been struck by the atmosphere at meetings. “You’re all so nice to each other,” one recalled a new arrival saying. “Someone even brought a cake to a meeting.”