Zack Polanski failed to show leadership over a motion passed by Green Party members declaring Zionism a form of racism, the party's former leader Caroline Lucas has told the BBC.
Lucas told Newsnight that the party's current leader, who did not attend the debate on the motion at the Green Party's annual conference in Brighton, "should have been there and he should have voted".
She described the motion, which defined Zionism as an ideology supporting "an ethnonationalist Jewish state", as "dangerous" and "divisive".
In the wake of the vote, Polanski has said he will not accept anyone being excluded from the Green Party "because of the religious or communal body they are part of".
Lucas's comments came after polls closed in the Holborn and St Pancras by-election, where Polanski is standing as a candidate.
Polanski did not vote on the motion after leaving the party's conference early to campaign in the north London constituency.
The Green leader has declined media interviews since the motion was passed on Sunday and did not answer questions from a BBC reporter about the motion earlier on Thursday.
The motion also backed a "single democratic Palestinian state in all of historic Palestine", which includes modern-day Israel.
Under Green Party rules, motions supported by members become official party policy, but will not necessarily be included in the party's manifesto at the next general election.
Lucas, who voted against the motion, said: "I would have hoped [Polanski] would have been a very strong critic of this motion."
She added that it was "flawed from top to bottom".
Asked if Polanski should stand down, she told Newsnight: "I think let's just see what happens in the coming days and weeks. Obviously he has been completely taken up with this by-election.
"I want him to speak now about what's happened, I want him to show the leadership that was frankly missing on Sunday.
"I think the jury is out about whether he will do that - but I very much hope he will."
In response to Lucas's comments, a Green Party source said: "There are ups and downs in politics and when people are passionate they disagree in the heat of a campaign.
"We do things differently - unlike Labour who use anonymous sources and resort to factionalism."