A key supporter of Nigel Farage and Donald Trump has provoked ridicule after using his Cornish castle to fly the flag of a prominent conspiracy theory identified by the FBI as inspiring rightwing violence.

John Mappin, who owns Camelot Castle at Tintagel, used the Victorian hotel to promote a rightwing hoax known as QAnon, which the US intelligence agency has said is a motivator for “domestic extremists” who want to carry out violence in the US.

Mappin is a central figure behind Turning Point UK, the British arm of the pro-Trump American student organisation which has been endorsed by a number of Conservative politicians, including the home secretary, Priti Patel, and Jacob Rees-Mogg, as well as Farage, the leader of the Brexit party.

Last June, Mappin was seen at a lavish Turning Point UK fundraising dinner, sitting opposite Farage.

The anti-fascist organisation Hope Not Hate dismissed Mappin’s flag stunt at Camelot and said that his Turning Point UK – which recently unveiled a range of merchandise celebrating Boris Johnson’s election victory – was increasingly peripheral. “Mappin is an eccentric figure, considered outlandish even by his fringe rightwing peers. This childish ploy is a weak attempt at getting attention for himself and his marginal Turning Point UK organisation, and is better off being ignored,” it said in a statement.

The QAnon conspiracy theory is based on posts made by a user on the 4chan and 8chan websites. Its advocates believe that Trump is locked in a battle with elements of the “deep state”, including individuals such as Hillary Clinton.

Some also believe that Trump is on their side: he pointed out a “beautiful baby” whose jumpsuit was adorned with the “Q” symbol at a US rally last year and retweeted an account that promoted QAnon.

However, Hope Not Hate said that despite attempts by figures such as Mappin, the conspiracy theory has yet to gain traction within the UK. “The QAnon conspiracy theories feed into the wider paranoia and distrust of the American far right but have little traction in the UK outside of a few peripheral circles,” it added.

Mappin is a devotee of Trump, having awarded him an “honorary Camelot Castle knighthood” in 2016 and the following year was photographed with the US president and the first lady, Melania, in Washington. He was contacted for comment.