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LONDON — British spy agency GCHQ has issued a rare and angry denial of a “ridiculous” allegation, repeated by White House press secretary Sean Spicer, that it eavesdropped on President Donald Trump during the election campaign.

The signal intelligence agency went public after Spicer quoted a Fox New analyst to support Trump’s so-far-unfounded claim that he was wiretapped by President Barack Obama.

A British government official went further, saying Friday that White House had promised not to repeat the claim — further undermining Trump's insistence that he is correct.

In a testy press briefing Thursday, Spicer cited former New Jersey judge Andrew Napolitano, who alleged on Fox News that Obama “went outside the chain of command” and used the British agency so that there were “no American fingerprints” on surveillance of Trump.

Spicer said it was one of many reports that suggest the president’s claims "merit looking into."

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It brought a swift, highly unusual and furious public response from GCHQ, which is broadly similar to America’s NSA and monitors global communications.

“Recent allegations made by media commentator Judge Andrew Napolitano about GCHQ being asked to conduct 'wire tapping' against the then president-elect are nonsense,” the agency said in a statement. “They are utterly ridiculous and should be ignored.”

Prime Minister Theresa May's spokesman told reporters that the UK government had "made clear" to the White House that the GCHQ claims should be ignored and had received assurances in return that they will not be repeated.

The White House said Friday that U.S. officials spoke with their British counterparts in an effort to explain Spicer's comments.

"(British) Ambassador Kim Darroch and Sir Mark Lyall expressed their concerns to Sean Spicer and General McMaster. Mr. Spicer and General McMaster explained that Mr. Spicer was simply pointing to public reports, not endorsing any specific story," the White House said in a statement, according to a press pool report.

As he left the East Room on Friday, Spicer told reporters he doesn't regret the allegations he read Thursday about the British intelligence agency being involved in such surveillance. He also said he did not apologize for those comments.

"I don't think we regret anything," Spicer said according to a pooled press report. "We literally listed a litany of media reports that are in the public domain."

The White House maintains Trump will ultimately be vindicated over his March 4 tweet that “Obama had my ‘wires tapped’ in Trump Tower,” despite a joint bipartisan statement from Senate intelligence committee leaders that they had seen no evidence.

Spicer emphasized that investigations were incomplete and that Trump had meant general surveillance when he wrote about wiretapping.

Napolitano repeated his theory on his personal website, saying the NSA had given GCHQ “the digital versions of all electronic communications made in America in 2016, including Trump's.”

“So by bypassing all American intelligence services, Obama would have had access to what he wanted with no Obama administration fingerprints,” he wrote.

Obama has already waved away Trump’s claim as “simply false.”

Rare on-the-record response from GCHQ to claims of wiretapping against Pres Elect Trump. "Nonsense...utterly ridiculous...should be ignored" — Rohit Kachroo (@RohitKachrooITV) March 16, 2017

How important an ally is GCHQ? Hayden wrote that in the event of a catastrophe, NSA's plan was to hand everything to the Brits. pic.twitter.com/GzddHktnfV — Susan Hennessey (@Susan_Hennessey) March 16, 2017