Westminster leader Ellie Chowns has called a Green Party motion defining Zionism as racism a "significant and damaging policy" and said she will work to reform the party's structures to prevent capture by extremists.

The motion passed at the Green Party conference earlier this month after members voted 1,022 to 813 in favour. The policy defines Zionism as an ideology supporting "an ethnonationalist Jewish state" and backs a "single democratic Palestinian state in all of historic Palestine", which includes modern-day Israel.

Speaking to BBC Radio 4's Today Programme on Saturday, Chowns said: "I don't know if there are extremists and racists in my party. But I want to make sure that my party isn't vulnerable to capture by extremists and racists." She described Zionism as a "contested concept" and said the motion did not represent the views of most Green Party members or the country.

The Green Party of England and Wales has 230,000 members. The motion was approved by just 0.4% of the membership, according to Chowns. She said this raised questions about party structures and policy-making processes.

Ex-leader Caroline Lucas has alleged the party was vulnerable to entryism, where people join to change its direction from within. Lucas called for the Greens to be made "a welcoming place for Jewish people" but said she was not blaming current leader Zack Polanski, who she said had done "great things" for the party.

Current leader Polanski, who is Jewish, said he would not have voted for the motion and would have supported an amended version instead. He did not remain at conference to vote, leaving early to campaign for the Holborn and St Pancras by-election, which he lost to Labour on Thursday.

The Board of Deputies of British Jews and the Jewish Leadership Council criticised the motion. They said: "Zionism is anti-racism, creating a refuge for Jews in their ancient homeland after millennia of persecution outside it."

Vivien Lichtenstein, leader of the Jewish Greens, called on Polanski to resign following his by-election defeat, saying it was a "big mistake" for him to have avoided debating the motion. Polanski told BBC Breakfast he would "absolutely" stay on as leader.

A Green Party spokesperson said members can bring and set policy, and the motion was one of dozens discussed at the party's largest-ever conference. Ashok Kumar, a Green Party member and part of Greens for Palestine, said the motion was about Palestinian liberation and he did not believe it was harming the party's image.