London Assembly member Zack Polanski has been elected leader of the Green Party of England and Wales by a landslide.

Polanski beat joint candidates, the Green MPs Adrian Ramsay and Ellie Chowns, by 20,411 votes to 3,705.

The result was greeted by whoops and cheers, with Polanski promising in his victory speech to “work every single day to grow this party” and paying tribute to his defeated rivals.

In a message to Sir Keir Starmer’s Labour Party, Polanski said the Greens were going to “replace you”.

Polanski, a former actor who was the party’s deputy leader, campaigned on an “eco-populism” platform and has promised to make the party “bolder” and more radical in its approach.

In his victory speech, he said his aim was to build a “green left”, adding the party had “an absolute moral responsibility in this moment” to stand for bold politics.

“We will nationalise the water companies. We will hold this Labour government to account,” he said.

“Because when we look at Keir Starmer and what this government have been doing, whether it’s the two-child benefit cap, the disability cuts, the genocide in Gaza, 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.”

Thousands of new members joined the party during the leadership campaign, taking its membership to 68,500.

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

This year’s contest was due to have been held in 2024, after 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.

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