A former Labour general secretary has defected to the Green party, in the latest sign that allies of Jeremy Corbyn are moving in large numbers to Zack Polanski’s party.

Jennie Formby, who managed the Labour party from 2018 to 2020, told the Guardian she had signed up as a Green party member and intended to campaign for it before May’s local elections.

Formby is the latest senior ally of Corbyn to defect to the Greens, even as the former Labour leader tries to establish Your Party, his own leftwing alternative to Labour.

The defection of figures such as Formby, the former Labour adviser James Meadway and the former North of Tyne mayor Jamie Driscoll offers the Greens a policy and organisational heft they previously lacked, but also threatens to distance the party from its environmental roots.

Formby said: “Zack and the Greens are not scared to talk about economic justice and tax increases. I am increasingly concerned by seeing the extent to which Labour is in hock to corporate sponsors.”

She added: “The Greens also have some important policies on workers’ rights, which is important to me – all these things made me want to join the Green party and I want to do everything I can to support them.”

She said had voted Green since 2022 but had joined the party as a member four months ago and intended to campaign for her local council candidate.

Formby, a former political director of the Unite union, took over as Labour’s general secretary in 2018, having secured the support of Corbyn and the then shadow chancellor, John McDonnell.

She quit that post in 2020, shortly after Keir Starmer became leader, saying that the change in leadership meant it was the right time to step down. She is now highly critical of Starmer’s leadership, saying he started to break his promises almost as soon as he became leader.

“Keir was elected on a bunch of things he immediately reneged on,” she said. “He does not have a set of principles which he is willing to stick to.”

She added that she had quit Labour in part over its treatment of MPs who signed a letter criticising the war in Ukraine and calling for a peace deal which recognised Ukrainian self-determination but also “addresses Russia’s security concerns”.

A Labour source said: “The Green party has the wrong answers for Britain. Whether it’s opposing housing and clean energy schemes across the country, or their lack of serious and credible proposals on the challenges facing working people, Zack Polanski’s party are not the answer.”

Polanski has reshaped the Greens since he became leader, focusing less on environmental issues and talking more about the economy, including taxes and the cost of living.

He recently announced his party would no longer focus on GDP as a measure of economic success but instead on people’s mental health, social cohesion and community welfare. He has campaigned in recent weeks for ministers to set aside billions of pounds to support households if energy bills rise due to the Iran war.

His strategy has seen the party jump in the polls from 10% to 16% and take the former Labour stronghold of Gorton and Denton. Meanwhile Green party membership has more than tripled, from 68,500 last September to 220,000 this month. The focus on economic policy has also brought over many of those who worked with Corbyn at the top of the Labour party.

As well as Formby, Meadway and Driscoll, others have joined in recent months, including Michael Chessum and Joe Todd, who worked for the leftwing organisation Momentum and the former Corbyn adviser David Prescott.

Chessum recently told the Financial Times: “[The Green party] is in the process of being re-founded. That doesn’t mean moving away from its environmental principles. It’s about becoming a mass party of the left.”

Formby denied that the Corbyn supporters who had left the Labour party were re-forming their movement in a new party. “I think this movement is new, and has a lot of energy,” she said. “I was very supportive of Jeremy, of course. But there is clearly an appetite for it – it’s something people are very excited about.”