Labour has suspended Jackie Walker, vice-chair of the pro-Corbyn group Momentum, over controversial comments about the Holocaust.
Leaked footage shows the campaigner saying at a party training event that she had not found a definition of anti-Semitism she could work with.
Ms Walker is also shown questioning why Holocaust Memorial Day was not more wide-ranging to include other genocides.
Labour said it did not comment on individual party memberships, but it is understood Ms Walker has now been suspended.
Labour MP John Mann said Ms Walker's comments were "unacceptable in a modern political party" by any standard.
He also said the comments had caused personal offence and had "inspired waves of anti-Semitic and racist backlash including Holocaust denial".
"Enough is enough," Mr Mann said. "Though she claims impunity for many reasons, Jackie Walker's behaviour is discriminatory, provocative, offensive and by any standard, unacceptable in a modern political party.
"Not only must she be expelled from the Labour Party immediately, but all those abusing others in supporting her must go too."
Ms Walker, who said she and her partner are Jewish, released a statement apologising for any offence.
In an interview with Channel 4 News, she said: "I certainly wouldn't call myself an anti-Semite.
"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 added that whoever leaked the footage "had malicious intent in their mind".
Former London mayor Ken Livingstone, who himself was suspended from Labour over alleged anti-Semitic comments, defended some of Ms Walker's comments, saying "there's a difference between ignorance and anti-Semitism".
The steering group of Momentum, which emerged from the campaign to get Mr Corbyn elected as Labour leader last year, meets on Monday.
A spokesman for the group said that it would be seeking to remove Ms Walker as its vice-chair.
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.
Jeremy Corbyn has denied Labour has a problem with anti-Semitism following a series of suspensions of party members over controversial comments.
A review carried out by Shami Chakrabarti found earlier this year that the Labour Party "is not overrun by anti-Semitism, Islamophobia or other forms of racism".