Andy Burnham issued a furious condemnation of the Green Party at Prime Minister's Questions this afternoon, blasting their recent 'Zionism is Racism' motion.
The motion, passed at Zack Polanski's Brighton conference earlier this month, called for Israel to be treated like apartheid South Africa, and was branded a plan to destroy Israel itself.
Asked by a senior Conservative MP this lunchtime whether he would now categorically rule out any Labour coalitions with the Greens following their adoption of the new policy, Mr Burnham was withering in his assessment of the hard-left party.
Mr Burnham said: 'Can I condemn the motion that passed unequivocally - it is wrong, it is antisemitic, and this side of the house will have nothing to do with it.
'We will always take the right response when it comes to partnerships we have with any other parties.
'We cannot support a partnership with a party that holds [that] position.'
The Daily Mail has asked whether Mr Burnham's emphatic opposition to deals with the Green Party applies to local councils, where there are 14 pacts between Labour and the Greens.
The motion has sparked civil war within the Green Party, and threatened to destabilise Mr Polanski's own leadership.
Three former leaders of the party - Caroline Lucas, Adrian Ramsay and Carla Denyer - all came out to oppose the motion, branding it divisive and warning it will alienate Jewish members.
Andy Burnham said he would rule out working with the Greens because of their new 'Zionism is Racism' policy
Zack Polanski is under mounting pressure over his failure to prevent his party passing the motion at their Brighton conference earlier this month
Green Party deputy Mothin Ali was supportive of the motion ahead of the Greens' conference
Mr Polanski himself missed the vote in Brighton, as he was campaigning in the Holborn and St Pancras by-election in North London.
This morning Nigel Farage called on Mr Burnham to rule out working with the Greens following the passing of the Zionism is Racism motion.
Writing for the Jewish Chronicle, the Reform leader said he would never tolerate 'sectarian politics... neither should Labour'.
'Britain has no shortage of political differences. We argue fiercely about tax, social care and energy. Those are all legitimate debates.
'Equating the existence of the world’s only Jewish state with racism is not. It is a descent into the oldest and ugliest of prejudices masquerading as progressive virtue.
'I will not tolerate sectarian politics or the importation of fundamentalism into our once vibrant democracy. Neither should Labour.'
A former Labour minister told the paper: 'I'm no fan of Nigel Farage, but he's absolutely correct here.'
At the weekend it emerged that the Greens might not be able to change their newly-adopted 'Zionism is Racism' policy for two years, despite Mr Polanski suggesting he wishes to ditch it at the 'earliest opportunity'.
The Standing Orders Committee would decide whether a new motion could be debated at the Greens' spring conference, scheduled for Leeds in March.
But its rules suggest a bid should be rejected if it 'significantly amends the principles passed in a policy motion or voting paper less than one year and nine months previously', according to the Telegraph.
That would mean no change is possible until Autumn conference in 2028.
Party sources told the Daily Mail there were too many 'variables' to say 'definitely' when the policy could be altered.
A Green spokesman said: 'We're a democratic party. Any member can submit a motion to change existing party policy, to be voted on by the membership.
'It is up to our Standing Orders Committee to rule motions going to conference in or out of order according to our constitution.'
The Campaign Against Antisemitism (CAA), along with a number of Labour MPs, have reported the Green Party to the Equalities and Human Rights Commission (EHRC)_ demanding an investigation into possible discrimination of Jewish members.
The CAA said they had concluded that there must be a probe on the basis the Greens have discriminated against Jewish members and/or associates, and continues to do so contrary to sections 101(2)(d) and/or 101(3)(d) of the Equality Act 2010.
Gideon Falter, Chief Executive of the CAA, said: 'The Green Party’s decision to adopt its "Zionism is racism" motion is racist and unlawful because it discriminates against Jews. Whether the Party believes itself to be above the law, or – by staggering incompetence – has not bothered to check the legal position, we will ensure that it faces the consequences of its actions.
'We are aware of various Jewish and Zionist members of the Green Party who now find their continued presence in the Party untenable, as well as many who have left the Party, and we are asking anybody in such a position to contact us confidentially so that we may examine further action against the Green Party.
'We will not stand idly by as extremists hijack a political party and make antisemitism its openly stated policy.'