Chief Rabbi Ephraim Mirvis has warned Labour that a vote in favour of new antisemitism guidelines would send a "message of contempt to the Jewish community", in an unprecedented letter to the party's ruling body.
It comes hours after an extraordinary letter from 68 leading British Rabbis, ranging from Liberal to strictly Orthodox to the party "antisemitism within sections of the Labour party has become so severe and widespread"
The Chief Rabbi urged the party's national executive committee to "follow the example" of the Parliamentary Labour Party, who voted on Monday night to adopt the full International Holocaust Remembrance Alliance definition of Jew hate.
He adds: "It is astonishing that the Labour Party presumes that it is more qualified than....in particular, the Jewish community to define antisemitism."
On Tuesday, the NEC will vote on whether to adopt Labour's new antisemitism code, which omits key parts of the IHRA definition, including how criticism of Israel can be antisemitic.
Rabbi Mirvis described this vote as "a watershed moment" and suggested those who vote against the full IHRA definition "will be placing themselves on the wrong side of the fight against racism, antisemitism and intolerance".
He also states: "Other groups might also legitimately ask if they will be next in having the prejudice they are subject to defined for them."
But in a sign that Labour General Secretary Jennie Formby was not conceding ground, a letter sent to all Labour MPs on Monday night claimed the new code was identical to the IHRA definition on all but one example.
Ms Formby wrote: "The only one of the IHRA examples - or rather one part of one example -that is not quoted or explicitly referenced in our code deals with claims about the state of Israel being a 'racist endeavour.'
"The wording in the IHRA example is open to different interpretations and run the risk of prohibiting legitimate criticism of Israel."
She added that Labour's alternative code "provides the necessary explanation to ensure legitimate criticism of Israeli policies is not silenced while not tolerating comments which deny Jewish people the right to self-determination or hold Israel to unfair standards not expected of other states".