The Green Party of England and Wales has been declared a racist organisation by the Board of Deputies of British Jews.
The Jewish body, which is the largest in the UK, was responding to the "Zionism is racism" policy motion passed at the Green Party's conference earlier this month.
Board of Deputies president Phil Rosenberg said a vote passed by 118 votes to one to declare that the Greens had, "by adopting an antisemitic platform, become a racist party".
A spokesman for the Greens insisted the party does not discriminate against people who hold Zionist views and said it rejected the Board of Deputies' "characterisation of this motion and our party".
They added the decision "will not stop us from speaking out against the genocide in Gaza and pushing for stronger sanctions against the Israeli government and its leaders". Israel has repeatedly denied its actions in Gaza amount to genocide.
The motion passed by the Board of Deputies, external urged other parties not to do deals with the Greens unless they rejected the policy.
Forming coalition deals with Greens at local or national level would be "failing to take antisemitism with the requisite seriousness", it said, unless the party's local leadership have rejected the policy and committed to overturn it.
Labour has already issued an ultimatum to local Green groups it is currently in coalition with, telling them to disown the motion or lose the party's support.
Rosenberg said: "By adopting an antisemitic platform, the Green Party of England and Wales has become a racist party. We will treat it accordingly.
"No political party has the right to tell Jews that a central part of our identity, rooted in thousands of years of history, faith and peoplehood, is racist.
"The Green Party's policy brands much of our community, including our synagogues and communal organisations, with that slur. It gives prejudice against Jews a political licence.
"The Green Party's leadership must now take responsibility for reversing this policy and confronting the antisemitism it has legitimised."
The motion backed by Green members at its party conference earlier this month said Zionism was an ideology supporting "an ethnonationalist Jewish state in historic Palestine to the exclusion and/ or domination of the non-Jewish population".
It was criticised by Jewish groups, as well as four Green MPs and two former leaders, and opposition political parties.
Responding when the Green motion was passed, the Board of Deputies and Jewish Leadership Council said in a joint statement that "Zionism is anti-racism, creating a refuge for Jews in their ancient homeland after millennia of persecution outside it".
Green Party leader Zack Polanski, who is Jewish, did not take part in the vote on the motion as he left the conference early to campaign in the Holborn and St Pancras by-election which was held days later.
He has since told the BBC's Sunday with Laura Kuenssberg that, while he would like to make amendments, he would not overturn the motion, which would be party policy until 2028.