The vice chairwoman of pro-Jeremy Corbyn campaign group Momentum has suggested she will not resign as calls for her to step down intensify over accusations of anti-Semitism.
Jackie Walker said "political differences" were underlying many of the calls for her resignation.
Ms Walker has come under fire after leaked footage of an anti-Semitism training event showed her saying she had not found a definition of anti-Semitism she could work with.
The footage also showed her questioning why Holocaust Memorial Day was not more wide-ranging to include other genocides.
Trade union boss Manuel Cortes has now called for Ms Walker to stand down, while reports have also emerged that Momentum's steering group is looking to remove Ms Walker from her post.
But Ms Walker defended her position on Channel 4 News when she was asked whether she had considered resigning given the outrage among some Jewish groups.
Ms Walker said: "Some other prominent Jewish groups, of which I'm a member of, think a very different thing.
"What we have to look at when we're talking about this subject, particularly at the moment, is the political differences that are underlying this as well."
Ms Walker said whoever leaked the footage from a Labour Party anti-Semitism training event "had malicious intent in their mind".
She also said she was anti-Zionist rather than anti-Semitic, adding: "I think Zionism is a political ideology, and like any political ideology, some people will be supportive and some people won't be supportive of it. That's a very different thing."
Ms Walker was previously suspended from the Labour Party for comments on social media saying Jews were the "chief financiers of the sugar and slave trade".
She was readmitted to the party after an investigation.
Ms Walker also posted comments on social media saying that proportionately more gypsies were killed by the Nazis than Jews, in terms of total of population.
She has had support from six Jewish Labour activists, who issued a statement saying Ms Walker had been subject to a "witch hunt".
Jeremy Newmark, the chairman of the Jewish Labour Movement, has said Ms Walker should resign from her position in Momentum.
Former Labour frontbencher Chris Bryant has also tweeted: "Just in case anyone wonders, I think Jackie Walker's comments are antisemitic and that she should be shown the door never to return."
Mr Cortes, general secretary of the TSSA trade union, said: "I am deeply saddened that a fellow member of our Labour and trade union family holds such anti-Semitic views.
"Jeremy (Corbyn) rightly spoke yesterday of the urgent need for our party to unite so we can take on the Tories.
"Frankly, I can't think we can do this when people who hold such abhorrent racist views are allowed to remain active within our party."
He urged Ms Walker to resign, or for both the Labour Party and Momentum "to act to get rid of her at once".
He added: "TSSA will seriously reconsider our union's support for Momentum if she is still in post by this time next week."