The White House is 'peddling falsehoods' with claims that GCHQ helped Barack Obama spy on Donald Trump, a former UK ambassador to Washington has warned.
Sir Peter Westmacott said the US administration was playing a 'dangerous game' and risked providing a 'gift to our enemies' by damaging intelligence-sharing.
The diplomat urged Foreign Secretary Boris Johnson to raise the issue when he visits Washington later this week.
The warning came after White House press secretary Sean Spicer sparked a row by citing a Fox News analyst's claim that Mr Obama had used UK listening agency GCHQ to bug Trump Tower.
Sir Peter Westmacott said the US administration was playing a 'dangerous game' by repeating claims about GCHQ
Mr Trump, seen arriving in Palm Beach on Friday, has said his press secretary was merely repeating claims by a Fox News analyst
The claim brought a rare public denial from GCHQ which described the suggestion as 'utterly ridiculous'.
Downing Street said it had secured an assurance that the allegation would not be repeated.
But at a joint press conference with German Chancellor Angela Merkel on Friday, Mr Trump said only that Mr Spicer had just been quoting retired judge Andrew Napolito who he described as a 'very talented legal mind'.
'We said nothing,' he said. 'I don't make an opinion on it,'
Sir Peter, who retired last year, said senior US officials were well aware the president was playing a 'dangerous game' which revolved around Mr Trump's 'famous reluctance to admit mistakes' combined with his suspicion of intelligence agencies.
He warned it risked undermining a relationship that was 'critical' to US-UK efforts to counter a range of threats including terrorism, Russian aggression, Chinese cyber-attacks, and the nuclear threat from North Korea.
The allegation that Barack Obama used GCHQ to bug Trump Tower has been dismissed as 'utterly ridiculous'
'The intelligence relationship between Britain and America is unique and precious. It is based on unquestioned mutual trust, between operatives and politicians on each side of the Atlantic,' he wrote in the Guardian.
'That is something both countries have taken for granted since the Second World War.
'Gratuitously damaging it by peddling falsehoods and then doing nothing to set the record straight would be a gift to our enemies they could only dream of.'
Sir Peter's comments will be seen as underlining the strength of feeling among the British intelligence and diplomatic communities at the actions of the White House.
GCHQ took the rare step of issuing a public denial after the White House cited the allegations. Pictured, the listening agency's hub in Cheltenham