Michael de Whalley, who was instrumental in setting up its West Norfolk branch in 2009, has given up his membership after more than 15 years with a blistering attack on the party.

He said: "I was always impressed that the Green Party seemed to be informed by science and evidence. The party has now moved away from that to more dogma and ideology."

Michael de Whalley (right) has sat on West Norfolk Council since 2019 (Image: Ian Burt)

The councillor said he became disillusioned with the direction Mr Polanski has taken the party in since he became leader last year.

Many more traditional members have been concerned about the Greens' increasing focus on left-wing economic policies, cultural topics like gender ideology, and foreign issues like Palestine, at the expense of concern for the environment and climate change.

Zac Polanski took over the Greens last year (Image: PA)

"There are a very significant number of older, deeper Greens who are looking on in horror - and I'm one of them," Mr de Whalley said.

The councillor, who is currently cabinet member for climate change and biodiversity at West Norfolk Council, singled out the party's recent economic policies and spending commitments for criticism.

"We are in an economic crisis and, thanks to the war in the Middle East, it's just got worse and could potentially impact the rest of this decade," he said.

“Going down a populist route for economics - saying 'I can spend money, I can borrow money irresponsibly, and it's okay, we'll cover it by increasing taxation' - it might sound good, but it's a recipe for disaster."

Mr de Whalley, who served with the RAF in Iraq in 2003, added: "Zack Polanski is looking at what pleases people and making promises he can't deliver. I can't be part of a party that is promising things that can't be delivered."

He added: “We've been really good at putting forward manifestos that stood up to scrutiny, but I don't believe that the economic directions currently do that.

“The Green Party is now moving to a left-wing populist stance. Our policies have less substance to them, whereas previously we were looking across the board."

The councillor, first elected in 2019, spent four years in opposition before joining West Norfolk’s cross party administration in 2023. 

He will now sit as an independent, remaining within the current administration.

The Green Party has surged in popularity since Mr Polanski became leader (Image: James Manning / PA)

Under Mr Polanski's leadership, the Greens have surged in the polls and membership is reported to have tripled to more than 200,000.

Last month, it won its first ever parliamentary by-election in Gorton and Denton, with a campaign that focused heavily on the war in Gaza, the government's proposed cuts to disability benefits and proposals for a wealth tax.

But the shift has brought to the fore deep tensions in the party between the more traditional, environmentally-focused members (so-called Deep Greens), those who are more left-wing (Watermelons - green on the outside, red in the middle) and even former Lib Dems who have joined (Mangos - green on the outside, orange in the middle).

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