A former CIA officer responsible for previously peddling false allegations played a prime part in the fake claim that Barack Obama secretly asked GCHQ to wiretap Donald Trump, The Independent has learned.

Larry C Johnson, who made bogus charges that Michelle Obama made a racist speech against white people and that former Secretary of State John Kerry had raped women while serving in Vietnam, has emerged as one of the key figures behind what has become an international diplomatic confrontation between the US and UK.

On 6 March, the week after Mr Trump first accused Mr Obama of being responsible for the wiretap, Mr Johnson “revealed” in an interview with Russian state sponsored network Russia Today that there was a conspiracy between US intelligence and “Britain’s own GHCQ (sic)” to derail Donald Trump’s election campaign. He said he had repeated this to Andrew Napolitano, a retired judge, who made it a basis for his own accusation against Mr Obama and GCHQ on Fox News earlier this week. The falsehood was then given further exposure by Sean Spicer, Mr Trump’s spokesman, at a White House briefing, on Thursday.

The revelation about Mr Johnson’s role in the extraordinary affair came as the Trump administration dismissed an account by Theresa May’s official spokesperson that they had apologised and pledged not to repeat the GCHQ claim.

Asked about the issue at a joint press conference with visiting German Chancellor Angela Merkel, Mr Trump replied: “We said nothing. All we did was quote a certain very talented legal mind who was the one responsible for saying that on television, I didn’t make an opinion on it. You shouldn’t be talking to me. You should be talking to Fox.”

Mr Spicer denied reports from No 10 that he had apologised. “I don’t think we regret anything,” he stressed. “As the President said, I was just reading off media reports.”

Rupert Murdoch’s Fox News, meanwhile, was busy distancing itself from the “very talented legal mind” Mr Napolitano. Anchor Shepard Smith said “Fox News knows of no evidence of any kind that the now President of the United States was surveilled at any time, anyway. Full stop.”

Mr Napolitano, who knows Mr Trump and has an apartment at Trump International Hotel & Tower in New York, was said to be lying low today. But Mr Johnson came forward to say that he was one of the sources for the GCHQ story. Mr Johnson maintained that his own knowledge of the matter came from the American intelligence community. “It sounds like a Frederick Forsyth novel,” he said.

Mr Johnson has been accused of mixing fact with fiction before. In 2008 he claimed on his blog that a tape existed of Michelle Obama “railing against whitey” at a church. Although he had not seen the tape himself, he said, “five other sources” had and it was being held by the Republicans “to drop at the appropriate time”. No such tape was released and no evidence was ever produced to prove its existence. The Obama campaign’s “Fight the Smears” website declared that the allegations were an invention.

In 2013, in another blog post, Mr Johnson falsely accused John Kerry of sexual assault, claiming that he had “raped some poor Vietnamese woman” in Vietnam. The assertion came from a TV debate in 1971 which had been edited and altered to make Mr Kerry say “I personally raped for pleasure”. When the manipulation was pointed out by readers of the blog he deleted the article. No apology was ever offered.