Zack Polanski called on the Greens to listen to the concerns of Reform UK voters as issued a plea to supporters of Nigel Farage's party.
Mr Polanski, who previously signalled support for former Reform voters to join the Greens, said party figures needed to understand why people were attracted to Nigel Farage’s party.
The London Assembly member was speaking at a conference organised by the left-wing campaign group Compass.
Also speaking at the event was former Green Party leader Caroline Lucas, who echoed Mr Polanski's comments.
Mr Polanski said: "I could stand here, and spend the rest of the evening talking about why Reform are bad, and I’d feel great. But there’s a really important distinction that needs to be made.
"Nigel Farage, the Reform MPs, the people who speak for them, are very different to people who might be thinking about voting for Reform. They are the exact people we need to be caring about.
"Because when we say people feel left behind, they don’t feel left behind, they have been left behind, by decades of austerity and by successive governments, by politicians who far too often speak to them like they’re stupid that’s if they’re even speaking to them at all."
He added: "Ultimately we need to reach out with this message of inequality and point out that when multimillionaires and billionaires are taking more money than ever before, the problem is not someone who is fleeing for their lives and might be travelling by small boat.
"The problem is flying above our head by private jet."
Zack Polanski has said he would welcome former Reform voters into the party
| GETTYSpeaking to The Guardian after the event, Ms Lucas, who served as the first ever Green Party MP from 2010 to 2024, said concerns of Reform voters are "perfectly legitimate."
She said: "We would disagree on the solutions they are reaching for, but when they have had the cost of living crisis for 20 years, and when they have had endless promises from other national governments that simply don’t deliver, you can’t be surprised that they act with a sense of desperation.
"If someone else is offering something that’s better, there is a sense that it’s worth giving it a try."
While she added she was glad the Greens had not "thrown the kitchen sink" at the Makerfield byelection, the former Brighton Pavilion MP stopped short of suggesting Sarah Wakefield, the party's candidate, should withdraw.
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Former Green Party MP Caroline Lucas stopped short of calling the party to withdraw
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Earlier this week, it was reported Mr Polanski's party had opted to mount only a minimal campaign in the Makerfield by-election, a move that significantly strengthens Andy Burnham's position as voters prepare to head to the polls on June 18.
While the party has chosen to put forward a candidate, resources will remain limited as attention shifts towards the anticipated contest to replace Mr Burnham as Greater Manchester Mayor.
The decision to scale back efforts in Makerfield comes amid internal disagreement over whether the Greens should be standing in the constituency at all.
Former co-leader Jonathan Bartley, who led the party between 2016 and 2018, had previously called for the Greens to step aside.
Andy Burnham has been handed a boost by the Green Party | GETTYA joint statement signed by councillors, activists and former party leaders has voiced unease about the decision to stand in Makerfield.
Rupert Read, a former councillor and author, is among those who have added their names alongside Bartley.
"Greens have always put their values and the climate and nature first," the statement declares.
"Which is why we approach the Makerfield by-election with some trepidation."