Jewish organisations have accused Ukip of embracing antisemitic conspiracy theories through the party’s links to a far-right US website that regularly attacks George Soros and has argued that the Pittsburgh synagogue attack could have been instigated by the US government.

The Board of Deputies of British Jews and the Community Security Trust (CST) has called on Ukip to dissociate itself from Infowarsafter it brought in one of the website’s editors as a member and used him to promote the party to younger people.

John Mann, the Labour MP who chairs the all-party parliamentary group against antisemitism, said Infowars was a “vile and dangerous” organisation.

It is led by Alex Jones, a web radio host who argues the 9/11 attacks, the 7 July bombings in London and the Sandy Hook primary school massacre were either faked or carried out by government-linked forces. Jones is being sued by Sandy Hook parents and has been banned from Twitter, Facebook and YouTube.

His de facto deputy at Infowars is Paul Joseph Watson, a Briton who has the title of “editor at large” for the website, and has regularly guested on Jones’s show.

In June, Watson was among a trio of YouTube personalities welcomed into Ukip as part of an attempt by its leader, Gerard Batten, to make the party more appealing to young voters.

Another was Mark Meechan, a self-described comedian and free speech activist who was fined this year for posting a video showing his girlfriend’s dog giving Nazi salutes in response to phrases including “gas the Jews”.

Watson’s membership was seen as a particular coup – he has 1.4 million YouTube subscribers – and after he joined, a video he made praising the party was featured prominently on Ukip’s website.

The association with Infowars has prompted renewed concerns about the direction of a party that remains the UK’s fourth-biggest by polling numbers, but under Batten has taken a more hard-right stance, mainly targeting Islam.

Infowars regularly carries disparaging stories about the supposed influence of Soros, the billionaire financier who is at the centre of antisemitic conspiracies about so-called “globalists” seeking to control the world.

Separately, Soros was among 14 prominent US figures to whom pipe bombs were posted this month. Jones has argued the bombs were a “false flag” plot.

In a broadcast after the Pittsburgh shooting last month, in which 11 worshippers were killed, he said the attack could have been instigated by “the deep state”. The government could have been “planning with groups they control to stage terror attacks” before the US midterm elections, he said.

Robert Bowers, the alleged gunman in Pittsburgh, made online posts about what he said was the Jewish control of the American government.

Jones and Watson insist they are not antisemitic, and Watson has not publicly endorsed many of Jones’s more lurid conspiracy theories.

But the Jewish groups said they were alarmed that Ukip should ally itself with the website. 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, which works against antisemitism in the UK, 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.”

Mann said: “In a climate of political uncertainty, conspiracy theories have been rising in popularity. Infowars is notorious for its part in feeding such conspiracies and at the root of many, if not all, of these myths is a hatred and suspicion of Jews.

“Any party serious about tackling antisemitism and racism would do well to cut all ties with infowars.”

A Ukip spokesman said: “Mr Watson is frequently the subject of attacks by actual antisemites because of his vociferous support for Israel. He is no more responsible for the comments of Alex Jones than Andrew Neil is for the comments of the director general of the BBC.

“This line of questioning, built on cynical supposition and innuendo, is a clumsy attempt to smear Mr Watson and Ukip for political gain. To comment any further would give it a credibility it does not deserve.”