Momentum's vice-chair, Jackie Walker, has come under fire for the comments she reportedly made at a Labour Party anti-Semitism training event.
Video footage obtained by Huffington Post showed Walker being jeered as she said: "Wouldn't it be wonderful if Holocaust day was open to all peoples who've experienced Holocaust?"
When told the day was indeed for all post-Second World War genocides, she said "in practice it is not circulated and advertised as such". She also said she had not heard a definition of anti-Semitism she could "work with".
On Twitter she later said "all holocausts are unacceptable - of course - and of all peoples".
Her comments have sparked calls for her resignation.
Holocaust Educational Trust chief executive Karen Pollock said: "Whilst Holocaust Memorial Day rightly and proudly commemorates the Holocaust and subsequent genocides, one has to wonder why Ms Walker takes issue with commemorating the mass extermination of Jews in its own right.
"The deliberate use of the term 'Holocausts' - plural - undermines and belittles the distinct nature of the tragedy itself, ignores that genocides are the result of diverse and unique factors, and also deprives the Jewish community of their collective memory."
The footage emerged after Jeremy Corbyn's speech, in which he denounced "evil" anti-Semitism.
The Labour leader said the party would take "firm action" against intimidation and vowed to fight against "prejudice and hatred of Jewish people".
A recent survey indicated that nine in 10 British Jews believe the Labour Party is too tolerant of anti-Semitism among its members and supporters.