Labour leader Jeremy Corbyn has apologised for appearing on platforms with people whose views he “completely rejects”.
Mr Corbyn’s statement was in response to the Times reporting that he hosted an event in 2010 at which a Holocaust survivor compared Israel to Nazism.
The event was held at the House of Commons on Holocaust Memorial Day.
Labour MP Louise Ellman told BBC Newsnight she was “absolutely appalled” to hear about Mr Corbyn’s involvement.
The Labour leader’s apology comes after one of his supporters in the party’s National Executive Committee suggested that Jewish “Trump fanatics” were behind accusations of anti-Semitism in Labour ranks.
Peter Willsman has apologised and said not all his remarks were accurately reported.
At the 2010 event in the Commons, Jewish Auschwitz survivor and anti-Zionist Hajo Meyer gave a talk entitled The Misuse of the Holocaust for Political Purposes.
Mr Meyer, who died in 2014 aged 90, compared Israeli policy to the Nazi regime.
The Times said that Palestinian activist Haidar Eid also addressed the meeting, saying: “The world was absolutely wrong to think that Nazism was defeated in 1945.
“Nazism has won because it has finally managed to Nazify the consciousness of its own victims.”
Mr Corbyn said views were expressed which he did not “accept or condone”.
He added: “In the past, in pursuit of justice for the Palestinian people and peace in Israel/Palestine, I have on occasion appeared on platforms with people whose views I completely reject.
“I apologise for the concerns and anxiety that this has caused.”
Analysis: Labour stuck in cul-de-sac of blame
By BBC political correspondent Ben Wright
Prominent Jewish voices inside and outside Labour are furious that the leadership seems to claim a clearer idea about what constitutes anti-Semitism than they do.
Neither the actions nor words of Jeremy Corbyn have managed to calm this argument down.
And while the government struggles to keep its plan for Brexit on the road, Labour appears to be stuck in a cul-de-sac of blame and recrimination on how it’s tackling anti-Semitism.
In July, Labour faced criticism over its new code of conduct on anti-Semitism, which critics claimed did not sign up fully to the International Holocaust Remembrance Alliance’s working definition.
Ms Ellman said she was “exceedingly disturbed” by the situation, adding that by diluting the definition of anti-Semitism, Labour had “made it possible to compare Israel with the Nazis”.
She added: “Jeremy is our leader, we want to be the next government, we need to change the way that this country is run to address the injustices in our society.
“But we can’t do that while we are engulfed in this crisis of the Labour Party’s inability to deal with anti-Semitism in its own ranks.”