One of Britain's leading spy agencies has angrily denied claims - suggested by the White House press secretary - that it helped former president Barack Obama spy on Donald Trump.

In a very rare public statement, a spokesman for GCHQ told ITV News that any suggestion it was involved in spying on Mr Trump "was utterly ridiculous and should be ignored”.

The strongly-worded denial came shortly after Sean Spicer repeated an unsubstantiated and explosive allegation - first made by a Fox News analyst - that Barack Obama used British spies to watch Donald Trump on his behalf to ensure "there were no American fingerprints on it".

Video report by ITV News Washington Correspondent Robert Moore

"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," Andrew Napolitano, a former New Jersey judge turned political commentator, said on Fox & Friends on March 14.

Mr Spicer directly quoted Mr Napolitano as he addressed reporters during a combative press briefing on Thursday.

The White House press secretary was attempting to justify President Trump's belief that Mr Obama placed Trump Tower in New York under surveillance in the run-up to last November's presidential election.

Mr Trump has come under substantial pressure, including from those within his own party, to either provide credible evidence or retract his incendiary accusation.

He has done neither nor signalled any desire to do so.

And the White House has now risked escalating the controversy even further by embracing a theory that Britain - one of America's closest allies - conspired with a president against a presidential candidate.

White House Press Secretary Sean Spicer. Credit: AP

Asked whether President Trump had raised the allegation of British involvement with Prime Minister Theresa May and whether it could impact the so-called "Special Relationship", Mr Spicer said: "It has not been raised."

GCHQ's decision to break with convention to publicly comment on a story indicates a clear annoyance within the agency at the White House's desire to elevate Mr Napolitano's claim.

“Recent allegations made by media commentator Judge Andrew Napolitano about GCHQ being asked to conduct 'wire tapping' against the then President Elect are nonsense. They are utterly ridiculous and should be ignored," the spokesperson said.