Left-winger Zack Polanski has been elected leader of the Green Party in a landslide victory that is expected to cause new headaches for Sir Keir Starmer. Mr Polanski is popular with some left-wing voters and activists who backed Labour when Jeremy Corbyn was leader but have now abandoned the party. His election paves the way for Greens to form a partnership with the new party launched by Mr Corbyn and former Labour MP Zarah Sultana.

Polls show that 10 percent of voters would vote Green if an election was held tomorrow. It’s possible this figure could fall once the new Corbyn-led party officially launches, but if the Greens and new party work together then they could eat into support for Labour. Jeremy Corbyn’s party is currently known as “Your Party” but a permanent name will be chosen at a conference of activists.

The other candidates were Adrian Ramsay MP and Ellie Chowns MP, standing on a joint ticket as potential co-leaders.

Mr Polanski promised action on the social and environmental issues such as the cost of housing and spiralling bills. Speaking after the leadership election result, he said it is time for an “honest conversation” about how we deliver affordable and safe homes, green jobs and free childcare.

"The planet is burning and so is the public’s patience.” Mr Polanski said. “People are rightly feeling let down and angry at Labour and are appalled by the rise of the far-right. People have been crushed by spiralling rents and bills while wages have not kept pace. For too long it’s felt like there has been no reprieve. But there is a way forward.”

He continued, “We need bold action, bold leadership. It’s time the super-rich paid their share. It’s time to tax extreme wealth and redistribute it to those struggling to live, to breathe, to survive.”

Elected with the largest ever number of votes in a Green Party leadership election, he said: "My pitch to the country offering a vision of a real radical alternative is only possible because it builds on the success of recent years. We stand as a party with our highest ever membership, highest ever polling average, highest ever general election result, highest ever number of MPs and Councillors. This is because of the amazing work we’ve done with my predecessors.

"We’ve come so far, but this is only just the start. It’s time to take this bold vision of social and environmental justice up a gear. I come into this role today with urgency, purpose, and a single mission: To build a country where no one is left behind.”

Throughout a sometimes fractious campaign, Mr Polanski has urged the Greens to be “bold” and pushed what he brands an “eco-populist” agenda, while his rivals emphasised a need to appeal to a wide range of voters.

Mr Ramsay, the incumbent co-leader and MP for Waveney Valley, and North Herefordshire MP Ms Chowns have accused him of using “polarising” language that only appeals to a “narrow segment” of the electorate.

Mr Polanski said: “They say I’m polarising but can’t name a single example. So let me give them one, it’s the 99% versus the 1%. If you’re not Shell or a billionaire, I’m on your side.”

Voting ran until August 30, with anyone who had joined the party by the end of July eligible to cast a ballot.

Mr Polanski’s insurgent campaign had encouraged people to sign up to the Greens ahead of the vote, leading to accusations of “entryism” from some of his opponents – accusations he has denied.

Unlike other parties, the Greens hold leadership elections every two years.

This year’s contest was to have been held in 2024, after Mr Ramsay and his co-leader Carla Denyer were elected for an extraordinary three-year term in 2021, but the poll was delayed in order to avoid a clash with the general election.

Ms Denyer decided not to stand for re-election in May this year.

The Green Party of England and Wales currently has four MPs.

The election had been seen as crucial for deciding the future of the party, which Mr Polanski seen as a candidate who could move it to the left. He has been an outspoken critic of Israel's war in Gaza, one of the reasons he is popular with some Corbyn supporters.