Jon Lansman has for weeks been lobbying Labour’s leadership to adopt the international definition of anti-Semitism with all its examples, Jewish News can reveal.

Sources close to the Momentum founder told Jewish News that he “regretted” the fact Jeremy Corbyn’s Guardian article “lacked a firm commitment” to include all 11 examples in the party’s code of conduct.

The intervention by close a close ally, whose Momentum organisation played a key role in propelling Corbyn to the leadership, piles pressure on the Labour leader to change course, just a day after Tom Watson urged full adoption of the International Holocaust Remembrance Alliance definition.

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Lansman is a member of the national executive committee’s working group on anti-Semitism, convened by general secretary Jennie Formby.

A source said: “Like many other leading party members close to the leadership, he recognises the necessity of agreeing that definition in order to be able to begin to rebuild trust with the Jewish community, whatever concerns we may have about the application of some examples.”

But they added Lansman stands by the party’s code and believes it adds “precision and clarity” to the process, increasing the ability to deal with those engaging in behaviour that IHRA says may be viewed as anti-Semitic.

Lansman backed the controversial code at last month’s NEC, having described it as a “gold standard” for dealing with anti-Semitism in a political party that demanded “a higher standard of behaviour than the IHRA examples”. He wrote in the Guardian last month: “I regret that for some Jewish communal organisations, the IHRA wording is so sacrosanct that it cannot be expanded and built on, contextualised and turned into a practical document for a party to enforce.

Neither the Board of Deputies or the Jewish Leadership Council were consulted ahead of the code’s adoption, and the Jewish Labour Movement was invited to a meting to give evidence, without warning it was part of a consultation. Along with others like Rhea Wolfson, Lansman’s concern over the inadequate consultation led the NEC to commit to consult further on IHRA.

Jewish News revealed last weeekend that Corbyn, who is facing widespread opposition to his stance from community leaders and his own MPs, is ready to include three examples that are reworded in the current code. They include comparisons between Israeli policy and the Nazis and allegations of dual loyalty, which the code currently seems only to be “wrong”.

But has made clear his belief that the final missing example – claiming Israel is a ‘racist endeavour’ has been used to restrict criticism of Israel. The Board of Deputies and Jewish Leadership Council made clear there can be no negotiation on adoption of any of the examples – but Corbyn has nevertheless expressed confidence that a solution can be reached.

In his Guardian article, Lansman had previously warned that the allegation of Israel being a ‘racist endevour’ was the example that ran the greatest risk of preventing legitimate criticism. “It cannot be right that one vaguely worded subset of one IHRA example can deny other oppressed groups their right to speak about their own oppression,” he said. “That doesn’t mean that there aren’t contexts in which claims about Israel being a racist endeavour are antisemitic or made with antisemitic intent. But the IHRA’s wording is not sufficiently clear. Labour’s code of conduct provides the necessary explanation.”

The definition is not the only issue on which Lansman has departed from the party line in recent days; it was also reported that he described the decision to pursue disciplinary action against Margaret Hodge and Ian Austin as a mistake.

He is also facing opposition within Momentum after pushing for the group to withdraw support for Pete Willsman as a candidate for the NEC elections, after his outburst denying anti-Semitism in the party was revealed. However, he also faced condemnation for continuing to campaign for Willsman’s election after witnessing the outburst himself, until a tape of the angry attack was published.

Meanwhile, seven leading anti-Semitism experts have insisted that any modification of the international definition of Jew-hatred is “no longer the IHRA definition”.

In a pointed intervention, members of the UK delegation to IHRA said: “Adding or removing language undermines the months of diplomacy and academic rigour that enabled this definition to exist. If one organisation or institution can amend the wording to suit its own needs, then anyone else could do the same.”

Signatories include survivor Sir Ben Helfgott, Karen Pollock and Alex Maws of the Holocaust Educational Trust, Olivia Marks-Woldman, of the Holocaust Memorial Day Trust, and Michael Newman from the Association of Jewish Refugees (AJR).

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