Green Party members have adopted a motion declaring Zionism a form of racism, in a move that has divided senior figures at the top of the party.

The party’s joint deputy leaders have been at odds over the subject, with Mothin Ali backing the move, and Rachel Millward describing the wording as “problematic”.

Activists endorsed the policy motion, which was put forward by a group of members at the party’s conference in Brighton, by 1022 votes to 813.

Discussion about the vote has dominated the conference, and exposed divisions between the party’s newly enlarged membership base under leader Zack Polanski and its MPs elected at the last election.

The motion adopted by Green members defines Zionism as an ideology in support of “an ethnonationalist Jewish state” and backs a “single democratic Palestinian state in all of historic Palestine”.

“The real, material, manifestation of Zionism has been the historic and ongoing dehumanisation, displacement, dispossession, disenfranchisement and destruction of Palestinians,” it says.

It has attracted criticism from some Jewish groups, including the Board of Deputies and the Jewish Leadership Council who wrote a letter to the party ahead of the vote.

In a statement on Sunday evening, the organisations called the motion an “attack on the Jewish community”.

“Zionism is anti-racism, creating a refuge for Jews in their ancient homeland after millennia of persecution outside it,” it said.

“Branding most British Jews and Jewish institutions ‘racist’ stokes anti-Jewish hatred after a year in which British Jews have been murdered attending their synagogue, stabbed in the streets, and seen their community buildings firebombed.”

Israel’s Foreign Ministry said the motion went beyond “the boundaries of legitimate political debate” and was “a stain on the history of the United Kingdom”.

Debate about the motion, as well as the conflict in Gaza, has featured heavily during the conference, which is taking place after a surge in its membership in the wake of Polanski’s election as leader in September last year.

Speaking after the vote, Lubna Speitan, the Green member behind the motion, argued its backers wanted to voice opposition to “racist systems of oppression”.

“We stand united against all forms of racism and discrimination – no exemptions,” she added.

Polanski himself did not vote in the debate, having left the conference early to campaign in Holborn and St Pancras, where he is standing to be the MP. A full list of candidates in the contest can be found here.