The Board of Deputies and the Community Security Trust have called on Ukip to dissociate itself from an American far-right website that spreads George Soros conspiracy theories and claimed the US government may have been behind the Pittsburgh synagogue attack.
In June, Paul Joseph Watson, who is a "editor at large" at Alex Jones' Infowars website, was welcomed into Ukip earlier this year as a member as part of an attempt by leader Gerard Batten to make the party more appealing to young voters.
Mr Watson has more than 1.4 million followers of his YouTube channel and he made a video praising Ukip after joining the party alongside Mark Meechan, a self-described comedian and free speech activist.
After the Pittsburgh attack, in which 11 people died, Mr Jones suggested it was the work of "the deep state", adding the government may have been "planning with groups they control to stage terror attacks".
In the past Mr Jones, a web radio host, has argued that the 9/11 attacks and the 7 July bombings in London were either faked or carried out by government-linked forces.
He also argued that pipe bombs sent to philanthropist George Soros and other prominent US figures in recent weeks were also a "false flag plot".
A spokesman for the Board of Deputies said it was very concerned at the party’s association with people “propagating conspiracy theories and dogwhistle antisemitism”.
He said: “We call on the party leadership to publicly and unequivocally disassociate themselves from these views and to expel any members who are found to hold them.”
A spokesman for the CST, a UK antisemitism watchdog, said: “The seepage of antisemitism and conspiracy theories into mainstream politics encourages hatred and undermines democracy.
"Rather than shunning these dangers, Ukip appears to be embracing them; and in the process is losing any claim it once had to be a respectable, mainstream party.”
John Mann, the Labour MP who chairs the all-party parliamentary group against antisemitism, said Infowars was a “vile and dangerous” organisation.
