The British intelligence agency Government Communications Headquarters (GCHQ) has dismissed a claim that British spies were employed to bug Trump Tower as "utterly ridiculous".
The accusation was made by US media commentator Judge Andrew Napolitano - then repeated by White House press secretary Sean Spicer.
Mr Spicer was speaking after the US Congress rubbished Mr Trump's claim the Obama administration "tapped his wires" before the election.
He said the president "stands by" his claim.
Mr Spicer went on to quote Mr Napolitano as saying: "Three intelligence sources have informed Fox News that
President Obama went outside the chain of command - he didn't use the NSA, he didn't use the CIA, he didn't use the
FBI and he didn't use the Department of Justice - he used GCHQ."
The accusation prompted a rare public statement from the UK intelligence agency.
The denial read: "Recent allegations made by media commentator Judge Andrew Napolitano about GCHQ being asked to conduct 'wiretapping' against the then president elect are nonsense.
"They are utterly ridiculous and should be ignored."
Privately a source told Sky News the agency was "furious" the claims had been made.
Such an attack on the White House by a British intelligence agency is almost unheard of.
To make matters more serious, the statement would have almost certainly been signed off by Downing Street.
Mr Trump has not substantiated the allegation that he was bugged.
In a television interview on Wednesday he said he would "very soon" give more details about what he is now calling surveillance - not wiretapping.
"You're going to find some very interesting items coming to the forefront over the next two weeks," he said.
The verdict of Congress was damning.
In a statement, Senate Select Committee on Intelligence leaders Richard Burr and Mark Warner said: "Based on the information available to us, we see no indications that Trump Tower was the subject of surveillance by any element of the United States government either before or after Election Day 2016."
(Additional reporting from IRN)