Jon Lansman, the founder of the influential Momentum group of Corbynite activists, is among those pushing behind the scenes for the NEC to adopt in full the international definition of anti-Semitism.
Mr Corbyn is also believed to have been advised by shadow chancellor John McDonnell, one of his oldest friends, to make concessions to resolve the issue that has engulfed the party over the summer.
The NEC meets a day before Labour MPs are due to hold a formal ballot on whether to adopt the International Holocaust Remembrance Alliance definition and all of its accompanying examples. A defeat for Mr Corbyn in the vote would leave him further exposed.
Mr Lansman’s intervention comes after signs of a split has opened in Momentum group over the anti-Semitism row.
Grass roots members, including a former aide to the Labour leader, are demanding more say in the way it is run after “loudmouth” Pete Willsman was dropped by the group for claiming anti-Jewish social media posts might be faked.
Matt Zarb-Cousin, a former Corbyn spokesman, said slates - where organisations ask members to vote for a group of chosen candidates - should be opened up to elections or proper consultations. “There’s a feeling among particulalry younger members of Momentum that we as an organisation endorsed this person but we did not have a say over who it was,” he told the Standard.
They have raised a petition calling for Momentum to “democratise as an organisation” to prevent national officers stitching up slates of approved candidates for positions in the party.
Mr Willsman was one of nine left-wingers backed by Momentum for positions on Labour’s ruling National Executive Committee. The slate was agreed by national officers in talks with smaller left-wing groups.
Labour dropped a probe this week into Dame Margaret Hodge for allegedly shouting at Mr Corbyn over his stance.
But the row shows no signs of abating, with Mr McDonnell now beinbg criticised over a speech in which he described Israeli attacks on Gaza as attempted genocide.
The Labour MP made clear he would not back down from his comments, releasing a statement which said he “takes pride” in his record of condemning brutality against Palestinians.
Mr McDonnell’s comments, in a speech in 2012, were highlighted by the Daily Telegraph, which also drew attention to his 2016 appearance on a platform alongside activist Jackie Walker, who was suspended by Labour for alleged anti-Semitism.
Speaking to a Unite the Resistance event at a time of intensive Israeli airstrikes on the Gaza enclave, Mr McDonnell said: “It’s absolutely critical now that we use every platform we can to expose what’s going on, which is effectively an attempt at genocide against the Palestinians.”
Karen Pollock, chief executive of the Holocaust Educational Trust, told the paper his remarks were “not only inaccurate, but irresponsible and deeply offensive”.
Mr McDonnell accused the Telegraph of “doing a number” on him.
He released a statement saying: “Our response: ‘John takes pride in and stands by his track record of forcefully and justifiably condemning brutal attacks on the Palestinian men, women and children of Gaza and will always stand up for victims of such disproportionate violence’.”