Donald Trump's official spokesman has repeated claims that Barack Obama used GCHQ to spy on him before he became president.
But the claim was dismissed as "nonsense" by the UK eavesdropping agency in a highly unusual public statement.
During a media briefing at the White House, presidential spokesman Sean Spicer drew reporters' attention to comments made earlier this week on Fox News TV by former judge Andrew Napolitano in relation to Mr Trump's controversial claim that wiretaps had been installed at his New York residence.
Detailing a long list of reports about the wiretap claims, Mr Spicer quoted 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."
In a surprise break from its normal practice of refusing to comment on allegations about its activities, a spokesman for GCHQ said: "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."
Liberal Democrat leader Tim Farron condemned Mr Spicer's retailing of the Napolitano claims as "shameful".
"Trump is compromising the vital UK-US security relationship to try to cover his own embarrassment," said the Lib Dem leader. "This harms our and US security."
Mr Spicer's intervention came shortly after the Senate Intelligence Committee released a statement saying they had seen no evidence to support the US president's claim - made in a series of Twitter posts earlier this month - that Mr Obama installed wire-taps at Trump Towers.
In a statement, the committee's Republican chairman Richard Burr and his Democrat counterpart 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."