Donald Trump remained unrepentant amid a diplomatic row with Britain on Friday, hours after US officials made a formal apology for accusing GCHQ of helping Barack Obama spy on the then presidential candidate.
Sean Spicer, the president's press secretary, repeated a claim on Thursday evening – initially made by an analyst on Fox News - that GCHQ was used by Mr Obama to spy on Trump Tower in the lead-up to last November's election.
The comments prompted a furious response from GCHQ, which in a break from normal practice issued a public statement: "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."
In a press conference with Angela Merkel, the German chancellor, on Friday afternoon, Mr Trump said his White House should not be blamed for quoting a Fox News analyst.
"We said nothing. All we did was quote a certain very talented legal mind who was the one responsible for saying that on television," he said.
"You shouldn't be talking to me, you should be talking to Fox."
He also doubled down on his unproven wiretapping allegation with a reference to 2013 reports that the US listened in on Mrs Merkel's phone calls.
He said that when it comes to wiretapping, "at least we have something in common, perhaps".
Intelligence sources had earlier told The Telegraph that both Mr Spicer and General McMaster, the US National Security Adviser, have apologised over the claims. "The apology came direct from them," a source said.
General McMaster contacted Sir Mark Lyall Grant, the Prime Minister's National Security adviser, to apologise for the comments. Mr Spicer conveyed his apology through Sir Kim Darroch, Britain's US ambassador.
Mr Spicer had earlier repeated claims that Mr Obama used GCHQ to spy on Mr Trump before he became president.
"He’s able to get it and there’s no American fingerprints on it," Mr Spicer said of the intelligence supposedly provided to Mr Obama by Britain.
"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."
Rick Ledgett, the number two at the US National Security Agency (NSA), said the claims were "arrant nonsense".
"Of course they wouldn't do it. It would be epically stupid," he told the BBC.
Mrs May's official spokesman said the White House has assured the Government that allegations that British intelligence services spied on Mr Trump would not be repeated.
The Government "made clear" to the US that the "ridiculous" claims should be ignored and received assurances in return that they will not be repeated, showing that the administration does not give them any credence, Mrs May's spokesman said.
Tim Farron, the Liberal Democrat leader, described Mr Spicer's comments as "shameful".
"Trump is compromising the vital UK-US security relationship to try to cover his own embarrassment," he said. "This harms our and US security."
A White House official said: "Ambassador Kim Darroch and Sir Mark Lyall expressed their concerns to Sean Spicer and General McMaster.
"Mr Spicer and General McMaster both explained that he was simply pointing to public reports and not endorsing any specific story."
Susan Rice, National Security Adviser for former president Barack Obama, tweeted:
The cost of falsely blaming our closest ally for something this consequential cannot be overstated. And from the PODIUM. https://t.co/lJ1Q1GR3lB— Susan Rice (@AmbassadorRice) 17 March 2017
The president is under increasing pressure to justify his claims, which his opponents charge calls the whole integrity of his administration into question.
In an attempt to provide credibility to the claims, Mr Spicer quoted from a series of articles which discussed surveillance.
He referenced comments made earlier this week on Fox News TV by Mr Napolitano in relation to Mr Trump's controversial claim that wiretaps had been installed at his New York residence:
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. What is that? It's the initials for the British intelligence-finding agency. So, simply by having two people saying to them president needs transcripts of conversations involving candidate Trump's conversations, involving president-elect Trump, he's able to get it and there's no American fingerprints on this. Putting the published accounts and common-sense together, this leads to a lot.
British officials were quick to rubbish Mr Napolitano's claims. A government source reportedly said the claim was "totally untrue and quite frankly absurd".
The British official told Reuters that under British law, GCHQ "can only gather intelligence for national security purposes" and noted that a US election "clearly doesn't meet that criteria".
Mr Spicer’s press conference on Thursday was held shortly after the senate intelligence committee published a statement saying they had no evidence for Mr Trump's claim, made on March 4, that Mr Obama ordered wiretaps on Trump Tower.