The White House has assured Britain's ambassador to the US that it will not repeat allegations that GCHQ spied on Donald Trump.
Ambassador Sir Kim Darroch spoke directly to Sean Spicer after the presidential spokesman repeated a claim that the British eavesdropping agency was used by Barack Obama to spy on Mr Trump before he became president.
In a rare public intervention, the allegations were denounced by GCHQ as "utterly ridiculous" - a position backed by senior UK Government officials, including Sir Kim and national security adviser Sir Mark Lyall Grant, in conversations with the US administration.
Prime Minister Theresa May's official spokesman said they had received assurances the claims will not be repeated - showing the US administration did not give them any credence.
"We have made clear to the administration that these claims are ridiculous and that they should be ignored and we have received assurances that these allegations won't be repeated," the spokesman said.
At a joint news conference with German Chancellor Angela Merkel, Mr Trump insisted Mr Spicer had just been repeating claims made by a legal commentator on Fox News, and that the White House had not offered an opinion.
"We said nothing. All we did was quote a certain very talented legal mind who was the one responsible for saying that on television. I don't make an opinion on it," he said.
He did not however back away from his claim that he was targeted by the Obama administration, suggesting he was the victim of the same sort of surveillance that was once alleged to have been used to monitor Mrs Merkel's calls.
"At least we have something in common, perhaps," he said.
The row followed a media briefing at the White House on Thursday, when Mr Spicer drew reporters' attention to comments made on Fox News by former judge Andrew Napolitano in relation to Mr Trump's claim that his New York residence had been bugged.
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 break from its normal practice of refusing to comment on allegations about its activities, GCHQ released a statement on Thursday night, saying: "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."
The statement was strongly endorsed by the chairman of the UK parliamentary Intelligence and Security Committee, Dominic Grieve, who said it showed the "strength of feeling" in the agency.
"The President of the United States is not able to task GCHQ to intercept an individual's communications," he said.
"Long-standing agreements between the 'Five Eyes' countries means that the US, UK, Canada, Australia and New Zealand cannot ask each other to target each other's citizens."
Mr Spicer's intervention came shortly after the US Senate Intelligence Committee said it had seen no evidence to support the US president's claim - made in a series of Twitter posts earlier this month - that Mr Obama had Trump Towers bugged.
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."