Farage, who has travelled to the US to support his friend on multiple occasions, said: “The ad didn’t say you’ll be going in your own free time, didn’t say you’ll have to pay your own air fare, which at the moment, by the way, are very, very expensive, it said you’re going to have free accommodation.

“If you look at the wording of that advert there is little doubt that is against American election law.”

Filings with the US Department of Justice reveal that Farage himself has received help three times since his election as an MP in July from a PR firm based in Pennsylvania, including with hotel costs on a trip to the USA.

The company, Capital HQ LLC, is run by Alexandra Preate, who used to be an aide to Trump’s former strategist Steve Bannon.

The filings show the firm paid more than $3,500 (£2,700) for Farage’s accommodation at the Republican National Convention in Milwaukee in July.

It also helped with his “perception management”, as well as public relations, travel and logistics.

Capital HQ disclosed it had also assisted with his appearance on a Fox News business programme in July, in which he was outspoken in support of Trump’s election prospects.

It also spoke to him in August about an “upcoming trip to the United States”.

The filings were made under US laws requiring anyone who acts on behalf of foreign “principals” engaged in political activity, such as seeking to influence policy or public opinion there, to register with the federal government.

The Clacton MP has declared £32,836 he separately received from a donor for flights and accommodation for a visit to the United States in July.

Asked about this on BBC’s Politics Live programme, Farage said he “hopped on somebody else’s plane, they gave me a free lift”, adding that he had declared it and “didn’t campaign at all”.

“I went in a purely personal capacity to offer my support after the first assassination attempt… as a friend of his and the family”.