Zack Polanski, a former actor, wants to make the Greens as noisy on the left as Nigel Farage's anti-immigrant Reform UK party is on the right as British politics fractures into a multi-party system.

"My message to Labour is very clear. We are not here to be disappointed by you, we are not here to be concerned by you, we're here to replace you," he told supporters in central London after being elected.

Polanski, who does not sit in the UK parliament, was seen as the more radical candidate compared to his defeated opponents, MPs Ellie Chowns and Adrian Ramsay, who had stressed continuity.

The party increased its number of MPs from one to four in the 650-seat parliament at last year's general election, which propelled Labour's Keir Starmer to the premiership.

It has the same number of MPs as Reform but has not been able to gain the same traction as Farage taps into discontent over irregular immigration, now the issue that most concerns Britons, according to surveys.

"We should never ape the politics of Nigel Farage and Reform. But what Nigel Farage has demonstrated is bold communications that can cut through," Polanski told reporters.

The Greens advocate higher taxes for the super wealthy, the nationalisation of the UK's water companies, and voice anger at what the party calls Israel's "genocide" in Gaza.

The party hopes to steal away voters angry at Labour's continuation of a child benefits cap, its moves to reform the welfare system, and its roll-back of some environmental pledges.

But it faces a threat on the left from former Labour leader Jeremy Corbyn, who has launched a new party, yet to be named.

Polanski, a member of the London Assembly, has repeatedly apologised for a 2013 article in tabloid The Sun in which the then-hypnotherapist promised to use the power of the mind to try to enlarge a female client's breasts.

Unlike other parties, the Greens normally hold leadership elections every two years, meaning Polanski may not lead the party into the next general election, expected in 2029.