The new Green leader, Zack Polanski, has said he would need to see “very strong arguments” before agreeing any electoral pact with Jeremy Corbyn, arguing that there is enough space on the left of British politics for both to thrive.

Polanski earned a resounding mandate to take the Greens in England and Wales in a more explicitly leftwing direction on Tuesday, winning 85% of member votes, a crushing defeat for the joint ticket of Adrian Ramsay and Ellie Chowns, two of the party’s four MPs.

Cheered by supporters at an event in London, the self-styled “eco-populist” promised to mimic the media-friendly tactics of Nigel Farage to tackle not just Reform UK but also Labour, saying the Greens under his leadership would not support a Keir Starmer government.

Calling Farage a “charlatan” who pretended to care about ordinary people, Polanski added, to cheers: “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 are here to replace you.”

While Ramsay and Chowns have been largely dismissive of the planned new party to be co-run by Corbyn and another former Labour MP, Zarah Sultana, Polanski said during the campaign that he was open to potential cooperation.

But speaking to the Guardian after the result was announced, Polanski said it was too early to say what, if anything, might come from this.

Asked if the Greens might hypothetically stand down candidates in a pact with the new party, he said: “It’s not my starting point, because at the moment I’m not quite sure what Zarah Sultana’s and Jeremy Corbyn’s platform is going to be.”

Polanski said he expected the Greens’ programme to be more radical and would need to be shown “very strong arguments” for any formal pact. A more likely outcome would be cooperation over which seats each party targeted.

He rejected the idea that the left of politics was a crowded field, highlighting that Starmer had not even condemned Farage’s “toxic tirade” about the proposed mass deportation of immigrants, merely saying he was “going to do it in a different way”.

“I don’t believe that the left of politics is actually a contested field,” he said. “Admittedly, if Jeremy Corbyn and Zarah Sultana do set things up, then it will be a busier place to be.

“I’m sick of being in media studios where I or the other Green is the only person who’s making the humanitarian argument. But if there are more personalities in our political spaces making those arguments, yes, at election time, there’s going to have to be conversations about how do we cooperate and make sure we’re getting out of each other’s way. But more widely in politics, I think more voices on the left is actually just a really good thing.”

Polanski presented himself as an insurgent voice during the election race, despite being deputy leader, arguing that the Ramsay-led tactic of gradually amassing councillors and then MPs was too timid and slow for an era in which Farage could win the next election.

Speaking to supporters after the result was announced, Polanski said he wanted the Greens to aim for more than 30 seats at the next election, saying it was “time to be really ambitious, and to be visionary” in getting the party’s message across to voters.

He ended his speech with an appeal for voters enthused by his message to sign up: “If you’re feeling hopeless, if you’re feeling in despair, if you’re feeling politically homeless, there is a political home for you.

“I promise you nothing will make you feel more inspired, more ready to get out there and more like we can turn our country around than joining the Green party. So join the Green party today.”

Corbyn congratulated Polanski in a post on X, saying: “I look forward to working with you to create a fairer, kinder world.”

Ellie Reeves, the Labour chair, also congratulated him but said the new Green leader “must level with the British people” about whether he backed continued Nato membership, after he said during the campaign that Donald Trump’s approach made this unfeasible.

Asked about the comments on Nato, Polanski said that while he maintained the alliance was untenable under a Trump-led US, he did not advocate immediate UK withdrawal, only if an alternative European defence alliance could be set up, a process he said could take years.