Andy Burnham has urged Zack Polanski, a Green Party member, to overturn a motion passed by the party that labels Zionism as a signifier of racism.
The premier curate said the motion, approved Sunday at the party's conference in Brighton, was "completely wrong" and showed "terrible judgement".
The motion defined Zionism as an ideology supporting "an ethnonationalist Jewish state" and backed a "single antiauthoritarian Palestinian authorities successful each of historical Palestine", which includes modern-day Israel.
In the aftermath of the vote, Polanski has said he would not judge anyone being excluded from the Green Party "because of the spiritual oregon communal assemblage they are portion of".
The Green Party member has declined media interviews since the motion was passed and did not reply to questions from a BBC newsman about the motion ahead of Burnham's comments.
It comes as polls open Thursday's Holborn and St Pancras by-election, in which Polanski is a candidate.
The motion has complicated the final days of Polanski's campaign in the contest, where the Greens have been seeking to overturn a significant Labour majority in the seat vacated by former Prime Minister Sir Keir Starmer.
It has sparked days of controversy, with 4 of the 5 Green MPs opposing it, its co-deputy leaders divided on supporting it, the Green deputy leader in Wales resigning, and condemnation from members and Green counterparts in Europe.
Polanski did not vote on the Zionism motion, having left the conference early to return to London for the by-election campaign.
An amendment he had backed to specify that the motion should not be used to target Jewish members attending synagogues that see themselves Zionist was voted down by members.
Before the vote, he had also said he wanted to see the reference to a single Palestinian authority changed, arguing: "I don't judge we arsenic a Green Party of England and Wales, oregon adjacent the British government, should beryllium deciding what the aboriginal colony looks like."
The Green Party has an internal ideology model under which official party policy is decided by members. However, these positions are not necessarily included in the party's wide-ranging election manifestos.
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