Unite general secretary Len McCluskey has said critics of Jeremy Corbyn used the issue of antisemitism within Labour in order to undermine him.
The union boss branded critics who sought to use antisemitism as "despicable" but added that the party "never handled the antisemitism issue correctly."
Speaking to the BBC's Andrew Marr Show on Sunday, Mr McCluskey said: "I'm absolutely convinced that there were those individuals who opposed Jeremy Corbyn's election right from the beginning, used the antisemitism issue - which I think is quite despicable that they did this on such an important subject - to undermine Corbyn, there's no doubt about that."
He added that the party had "never handled the antisemitism issue correctly” and that "we should have done things quicker."
The Equality and Human Rights Commission has been investigating Labour’s response to accusations of antisemitism among its members since last May.
It is investigating whether Labour has "unlawfully discriminated against, harassed or victimised people because they are Jewish".
In another interview on Sunday with Sky News' Sophy Ridge, Mr McCluskey said Labour's election defeat was "virtually, solely down to Brexit".
Mr McCluskey also told Marr that he offered “continued dialogue” with the Jewish Labour Movement over issues of antisemitism.
In response, Jewish Labour Movement’s national secretary Peter Mason rejected this claim with a series of tweets about JLM's relationship with the union boss.
Mr Mason wrote that Mr McCluskey “certainly hasn’t supported us. When it was clear his mind was closed and agenda fixed, we figured there was little point trying to explain anti-Jewish racism to a ‘lifelong anti-racist'."
Thread: Len McCluskey hasn’t offered “continued dialogue” with @JewishLabour and certainly hasn’t supported us. When it was clear his mind was closed and agenda fixed, we figured there was little point trying to explain anti-Jewish racism to a “lifelong anti-racist” #Marr
— Peter Mason (@_petermason) January 26, 2020