Momentum vice chair Jackie Walker has been axed from the role after making a series of controversial comments on the Holocaust.
Footage leaked from a training event showed Ms Walker 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.
A statement released by Momentum denied that Ms Walker's comments had been anti-semitic, but they were considered offensive given that they were made on Holocaust Memorial Day.
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The statement reads: "Momentum's Steering Committee has voted, seven to three, to remove Jackie Walker as its Vice Chair, a position it elected her to.
"She remains a member of Momentum and its Steering Committee.
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"Jackie's actions at Labour Conference, and her subsequent Channel 4 interview, and by not undertanding concern caused by her statements, have led the Steering Committee to view her behaviour as irresponsible and lose confidence in her as vice chair."
The statement added that Momentum felt that Ms walker could have done more to explain her comments.