Mr Williamson, the MP for Derby North and an outspoken ally of Jeremy Corbyn, signed the Change.org petition, tweeting a link with the caption “Hands Off Gilad Atzmon – Sign the Petition!”
He later deleted the tweet and apologised, saying: "Earlier today I tweeted a petition about an Islington Council ban against the Blockheads performing with their chosen line up. The council has blocked jazz musician Gilad Atzmon from playing with the group.
"Since then I've learned that Atzmon, a former Israeli soldier, is not confined to the jazz world. I am told that in various blogs and in speeches he has adopted antisemitic language.
"I wasn't aware of this until after I tweeted the petition. As soon as I was informed, I deleted the tweet. I've always condemned all forms of racism, including antisemitism, and strongly disassociate myself from Atzmon's antisemitic views.
"I therefore apologise for tweeting this petition and any distress or offence it may have caused."
Anti-racist advocacy group Hope Not Hate has said Mr Atzmon is “an antisemite who has promoted the works of Holocaust deniers”, while the Community Security Trust described Mr Atzmon book, the Wandering Who?, as “quite probably the most antisemitic book published in this country in recent years”.
Last year at the Reading International Festival, he also claimed that the Grenfell Tower tragedy was the responsibility of “Jerusalemites” who were “following mitzvot”.
He has also been accused of posting antisemitic tweets, including one which appeared to endorse an “Anne Frank Halloween costume”, and has said “I despise the Jew in me and detest the Jew in you”.
Human rights lawyer Adam Wagner called Mr Atzmon "an antisemite and Holocaust denier".
Despite Mr Williamson's apology, the Jewish Labour Movement reiterated its call for him to be suspended from the party.
JLM chair Ivor Caplin said: "Gilad Atzmon is an antisemite.
"Chris Williamson is a Jew baiter. Supporting Atzmon is baiting Jews. And Chris Williamson has been baiting Jews in the party for too long unchecked. No more ifs or buts.
"We expect Chris Williamson to be suspended from the Labour Party today."
A Labour Party spokesperson said: “Gilad Atzmon is a vile antisemite. Chris Williamson has said he was not aware of Atzmon‘s appalling views and rightly apologised for his tweet.”
Mr Atzmon’s writings include a 2009 blog, Tribal Marxism for Dummies, in which he said that while “Marxism is a universal paradigm, its Jewish version is very different. It is there to mould Marxist dialectic into a Jewish subservient precept".
He also argued that it is merely a "Judeo-centric pseudo intellectual setting which aims at political power", and that "Jewish Marxism is there to … stop scrutiny of Jewish power and Jewish lobbying".
A number of left-wing and pro-Palestinian organisations have distanced themselves from Mr Atzmon, and he has been uninvited from a series of book-promotion events.
In the wake of Mr Williamson’s endorsement of Mr Atzmon, 32 university Labour clubs signed an open letter urging the party to “withdraw the whip… until he listens to the concerns of the Jewish community and properly educates himself about antisemitism”.
It read: “Today Chris Williamson’s actions have again shown a complete lack of respect for the Jewish community. He repeatedly platforms and supports antisemites and takes no responsibility for his actions. We wholeheartedly condemn this.
“We stand in solidarity with the Jewish community, alongside groups such as the Jewish Labour Movement, who have repeatedly condemned Williamson’s antisemitic apologism. Our Labour values mean nothing if we don’t stand up against all forms of racism and make it clear that it has no place in our party.
“It’s disappointing that the Labour Party have not yet taken appropriate action.”
Earlier this year he was ordered by the High Court to apologise to Gideon Falter, the chairman of Campaign Against Antisemitism, after accusing him of personally profiting from fabricating antisemitic incidents.
