Boris Johnson has told ITV News he would "say goodbye" to President Trump if the White House asked for the NHS to be included in trade talks.
The prime minister said he would "walk away" from negotiations, agreeing the National Health Service is "more important" than a trade deal, before adding "there is no point in continuing the conversation".
He continued: "Why on Earth would we do something as stupid as that? We are going to keep the NHS, properly fund it."
The Conservative Party has repeatedly been attacked for its funding and position on the NHS, with Labour saying it has "proof" the party plans to put the NHS "up for sale".
It comes as a 451-page dossier showed talks, held when Theresa May was in Downing Street, saw UK officials discuss the NHS with their American counterparts.
Labour leader Jeremy Corbyn on deplored the talks, saying proposed "longer patents can only mean one thing - more expensive drugs". It claimed on Thursday a trade deal with Washington would "threaten the environment".
Boris Johnson says he has 'never tried to deceive the public' and believes race to Number 10 is 'very tight'
Mr Johnson said he has "never tried to deceive the public."
Asked by ITV News' Paul Brand if he had ever lied in his political career, he responded: "Absolutely not."
He went on to add: "I've always tried to be absolutely frank."
Mr Johnson conceded he had "got things wrong" and "may have been mistaken" but claimed he had never knowingly said something that wasn't correct.
Despite his claims, Mr Johnson was sacked by then-Conservative Party leader Michael Howard in 2004 for lying about an affair.
Lord Howard, who led the Tories between 2003 and 2005, sacked the then-shadow arts minister over a misrepresentation.
Speaking about the forthcoming General Election, Mr Johnson said he believes it to be "a very tight race".
He conceded he does not think his party is heading for a big majority, despite recent polls showing the Tories could secure more seats in the Commons than rivals.
Mr Johnson went on to admit he has "made mistakes".
"I'm not going to pretend that in my political career I have not said or done things that have caused offence," he said.
"I have certainly made mistakes."
Speaking about mental health, he said his solution to "feeling down" is to "work hard".
He told ITV News: "The best answer I can give you is that, insofar, as I have ever felt down, and everybody feels down from time-to-time, that the best answer is to work hard - for me."