The Chief Rabbi has warned Labour’s ruling body that failing to toughen up its new code of conduct on anti-Semitism would place it on the “wrong side of the fight” against racism and intolerance.

In a letter as its members prepare to meet, Ephraim Mirvis said it was a “watershed moment” for the party and urged it to “make the right decision for Britain”.

The National Executive Committee is expected to rubber-stamp a new code that was approved by a sub-committee when it meets on Tuesday.

It was drawn up in the wake of protests by Jewish groups outside Parliament earlier this year and states explicitly that “anti-Semitism is racism. It is unacceptable in our party and in wider society”.

But the documents stops short of signing up in full to the International Holocaust Remembrance Alliance’s (IHRA) working definition of anti-Semitism.

And it insists that criticism of the state of Israel and its policies should not automatically be regarded as anti-Semitic, and makes clear that even “contentious” comments on this issue “will not be treated as anti-Semitism unless accompanied by specific anti-Semitic content … or by other evidence of anti-Semitic intent”.

Mr Mirvis said it was “astonishing that the Labour Party presumes it is more qualified” to define anti-Semitism than the Jewish community and the governments around the world that accept the IHRA definition.

“Adoption of Labour’s new alternative to the internationally accepted HRA definition will send an unprecedented message of contempt to the Jewish community,” he added.

“Other groups might also legitimately ask if they will be next in having the prejudice they are subject to defined for them.”

The code explicitly endorses the IHRA’s working definition of anti-Semitism and includes a list of behaviours likely to be regarded as anti-Semitic copied word-for-word from the international organisation’s own document.

But it omits four examples from the IHRA list:

– Accusing Jewish people of being more loyal to Israel than their home country;

– Claiming that Israel’s existence as a state is a racist endeavour;

– Requiring higher standards of behaviour from Israel than other nations; and

– Comparing contemporary Israeli policies to those of the Nazis.

Labour insists that while the examples are not reproduced word for word, they are covered in the new code.

At a meeting of the Parliamentary Labour Party on Monday, MPs and peers overwhelmingly backed calls with a show of hands for the full IHRA deal to be adopted.

Although the vote does not bind the leadership, it shows the strength of opposition to the policy in its current state.

A Labour Party spokesman said: “These are the most detailed and comprehensive guidelines on anti-Semitism adopted by any political party in this country.

“They adopt the IHRA definition and contextualise and add to the working examples to produce a practical code of conduct that a political party can apply in disciplinary cases.”