Zack Polanski has vowed to stay on as Green Party leader after his by-election defeat, as he faced pressure from within his own party to resign.

The leader of the Jewish Greens, Vivien Lichtenstein, said on Friday he should resign, adding it was a “big mistake” for him to have dodged a debate about a party conference motion that declared Zionism a form of racism.

Lichtenstein’s comments come after two former Green leaders accused Polanski of failing to show leadership over the Zionism row.

Polanski, who came second to Labour overnight in the Holborn and St Pancras by-election, told BBC Breakfast he would “absolutely” stay on as leader and he “of course” had not questioned that.

The Greens had hoped to take the seat, previously held by former Prime Minister Sir Keir Starmer, after a by-election success earlier this year.

Polanski lost out in Thursday’s contest in London to Labour candidate Sagal Abdi-Wali – who secured 45% of the vote to his 33% – but the Green vote share was up by 22 percentage points on the 2024 general election.

Following the by-election, Lichtenstein was asked whether it would be right for Polanski to continue as leader of the party. She told Times Radio: “No, I think he should resign.”

Earlier in the week, Lichtenstein, who has been a member of the Green Party for nearly three decades, told Radio 4’s Today programme it was “quite possible” she would leave the party after the Zionism row.

Speaking to the BBC’s Victoria Derbyshire on Friday, she said she would not resign immediately because she is having conversations about how to “detoxify” the party.

When asked if she had confidence in Polanski’s leadership, Lichtenstein said: “At the moment, no I don’t.” She added that she was “very disappointed” in him.

Calling on Polanski to apologise, she said: “I don’t think that any Jew in the party is going to be in any danger…

“There just might be some unpleasant people who will direct some unpleasant comments, and some of us are going to have to deal with them robustly.”

Her criticisms were similar to those of former Green Party leader Caroline Lucas who said Polanski should have attended the vote on the conference motion on Sunday.

The motion, which was passed by rank-and-file members to become official party policy, defined Zionism as an ideology supporting “an ethnonationalist Jewish state” and backed a “single democratic Palestinian state in all of historic Palestine”, which includes modern-day Israel.

Adrian Ramsay, a Green MP and former co-leader, declined to back Polanski on Friday, and said he was now working to reverse the motion.

“A lot depends on what happens in the days and weeks to come,” he told the BBC’s Politics East, when asked if Polanski should still lead the party.

“[He] did not show leadership I would have been looking for.”