The head of the Jewish Greens has said she may leave the party after it voted to adopt a policy that equates Zionism with racism and calls for Israel to be replaced by a single Palestinian state.

Vivien Lichtenstein said that while opinion among Jewish members of the party was split on the motion passed at its annual conference in Brighton on Sunday, she said she believed it was unhelpful and it was “possible” she might quit.

It comes as anger grows among some in the party about the decision of Zack Polanski, the leader of the Greens in England and Wales, to miss the vote and to encourage supporters to do the same so they could instead campaign for him in Holborn and St Pancras before the parliamentary byelection for the seat next week.

Polanski sought to have the conference postponed so the party could focus on his efforts to win Keir Starmer’s former seat. When the Greens’ executive committee refused this, the party contacted members, encouraging them to get refunds on their conference tickets to campaign.

Sunday’s binding policy motion was passed by 1,022 votes to 813, and some Greens say more effort by Polanski could have stopped it.

One long-standing member said: “There’s so much anger at his disinterest in such a critical issue. You can’t run a party on vibes alone.”

The motion was opposed by four of the Greens’ five MPs, and is expected to face a legal challenge under equality laws. Polanski did not take a side but justified the motion’s broad argument by pointing to the death toll in Gaza.

Speaking on BBC Radio 4’s Today programme, Lichtenstein condemned the motion: “I don’t think it’s helpful, and it doesn’t make any difference whatsoever to Palestinians.”

Asked if she would remain in the party, she said: “I haven’t decided yet. Certainly it’s quite possible that I will leave the party, and I’m giving it a little time, and to talk with colleagues.”

The motion says Zionism – the movement for a Jewish state – should be “treated as any other form of racism”. It calls for Israel to be replaced by a single Palestinian state “with equal rights for all”.

Some Jewish Greens have expressed concern that this will mean they could face disciplinary action for belonging to Jewish organisations that officially support Zionism, even synagogues. Supporters of the motion, which was led by a group called Greens for Palestine, have said this would not happen.

Asked about this issue, Lichtenstein said she was less worried about being personally targeted than about the potential impact on organisations. She said: “I’m not concerned about people accusing me of supporting Israel’s right to exist, and therefore, I’m a racist, which of course is not the case. I’m just concerned that this is a very bad motion …

“What concerns me is that the party might accuse UK organisations, Jewish organisations, and also Israeli-Jewish and joint Arab-Jewish organisations, of being Zionist and therefore racist. I support an awful lot of those sorts of organisations that are working against the occupation, and even go out on the ground to assist Palestinians.”

The idea of calling for a single Palestinian state was not an issue for all Jewish Greens, Lichtenstein said, but party policy had been that it should be “up to Israelis and Palestinians to choose their future, one state or two”.

The chief rabbi, Ephraim Mirvis, described the motion as “poisonous antisemitism” and a “threat to British Jews and to Britain”.

He said he was “appalled” by the motion, claiming the Green party had resurrected a “discredited and dangerous slander” first promoted by the Soviet Union.

“All decent people will see this for what it is: poisonous antisemitism, a threat to British Jews and to Britain,” he said.

Israel barred Polanski from entering the country in retaliation for the motion, which the Israeli foreign ministry said “constitute[d] a clear antisemitic text”.

It is understood some Green members are consulting lawyers over whether the policy could be challenged for breaching equalities legislation.

Adam Wagner KC, the barrister who led a case that concluded Jeremy Corbyn’s Labour party broke equalities laws over antisemitism, said such a case would have a good chance of success. “I think there is a strong risk that the motion as passed contravenes the Equality Act 2010, which applies to political parties, as it will discriminate against and harass people who hold Zionist beliefs and Jewish people more generally,” he said.