Washington (AFP) - The White House on Thursday cited unproven media reports that President Barack Obama asked Britain's signals intelligence agency, GCHQ, to monitor Donald Trump in order to "make sure there were no American fingerprints."
Speaking from the White House podium press secretary Sean Spicer quoted at length from a Fox News report, which alleged Obama had used GCHQ to dodge US legal restrictions on monitoring US citizens.
The story was one of several offered by Spicer as evidence to support the president's explosive claims that Obama had moved to "tap my phones."
In a series of tweets on March 4, Trump accused Obama of a "Nixon/Watergate"-like plot that would almost certainly break US law.
"Terrible! Just found out that Obama had my 'wires tapped' in Trump Tower just before the victory. Nothing found. This is McCarthyism!" one tweet read.
Members of Congress from both parties who are investigating the claims have found no evidence to support them.
In the Fox report -- which came almost two weeks later -- Andrew Napolitano claimed that "three intelligence sources have informed Fox News that President Obama went outside the chain of command" to order the tap.
"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," Napolitano said, adding that Obama used GCHQ.
Spicer's citation, in front of the White House seal, raised some eyebrows in London and at the Cheltenham-headquartered agency, which has worked closely with US intelligence for decades.
"Recent allegations made by media commentator Judge Andrew Napolitano about GCHQ being asked to conduct 'wire tapping' against the then president-elect are nonsense," one GCHQ spokesperson told AFP.
"They are utterly ridiculous and should be ignored."
Britain and the United States are -- along with Australia, Canada and New Zealand -- part of the "Five Eyes" intelligence sharing alliance forged from the embers of World War II.