Zack Polanski’s allies are mounting an urgent defence of the Green Party leader as he faces intensifying criticism over an anti-Semitism controversy and growing uncertainty about his political future.

The pressure follows the passage of a “Zionism is Racism” motion at the Greens’ annual conference, a policy opponents say amounts to calling for Israel’s destruction.

Chief Rabbi Sir Ephraim Mirvis added his voice to the criticism, warning that the motion represented a “threat to British Jews and to Britain”.

Mr Polanski, who was refused entry to Israel last night, attempted to distance himself from the policy by joining the Green Party’s five MPs in condemning it. However, his response has done little to quell anger over what critics describe as a failure of leadership.

He has also been accused of political cowardice after opting not to attend the vote in Brighton, instead campaigning in Holborn and St Pancras before next week’s by-election, in which he is standing.

Mr Polanski is expected to be defeated by Labour candidate Sagal Abdi-Wali in Holborn, but his supporters have sought to lower expectations by presenting a relatively narrow margin as a possible measure of success.

Political commentator Aaron Bastani claimed Mr Polanski was being targeted by a “choreographed effort” aimed at forcing him to resign following Thursday’s conference vote.

“If the Greens get within 3,000 votes of Labour, in the middle of the Burnham bump — in the seat where (Keir) Starmer, (Peter) Mandelson and (Alastair) Campbell live — then it’s clear they’re here to stay,” he said.

Former Labour MP Zarah Sultana, who is now part of the hard-Left Your Party, wrote on X: “Reminder: anti-Zionism is not anti-Semitism. The smears won’t work.”

Mr Polanski’s failure to confront hardline pro-Palestinian activists within his party may also have damaged the Greens’ prospects of joining a centre-left “rainbow coalition” in government.

Andy Burnham said last week that he wanted to replace the UK’s electoral system with proportional representation, a change that could make coalition governments more common.

That prospect has prompted concern over whether a future administration could include Green MPs.

Paul Ovenden, a former aide to Keir Starmer, wrote on X: “Last week, the Green Party was offered the biggest opportunity it has ever had, in the form of electoral reform.

“Today, they have made it impossible for them to be included in any administration.”

The motion passed in Brighton by 1,022 votes to 813.

It formally describes the Greens as an “anti-Zionist party” and states that Zionism — the movement focused on establishing and safeguarding a Jewish state in Israel — should be “treated as any other form of racism”.

The policy also backs the creation of a “single democratic Palestinian State in all of historic Palestine with Jerusalem as its capital”.

Supporters chanted ‘say it loud, say it clear, Zionists are not welcome here’ as the campaigners behind the motion, Greens for Palestine, gathered to speak to the media.

Sir Ephraim 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 accused the Green Party of legitimising ‘the destruction of the State of Israel’ with a ‘clear antisemitic text’.

Foreign minister Gideon Sa’ar declared that Mr Polanski was now barred from entering Israel.

‘Today, the Green Party in Britain adopted a resolution equating Zionism with racism, legitimising armed struggle against Israel, and calling for the establishment of a Palestinian state ‘from the river to the sea’ – meaning the destruction of the State of Israel,’ the Israeli foreign ministry said.

In a statement with Green MPs and his deputies, Mr Polanski said there is ‘no conflict between supporting the rights of Palestinians and the rights of Jewish people’.