The United States has made an official apology to Britain for bizarre claims Barack Obama may have used UK spies at GCHQ to listen to Donald Trump's calls.

White House Press Secretary Sean Spicer has personally apologized after he repeated the claim in an official briefing, triggering a major diplomatic incident.

It prompted a rare and furious response from GCHQ, which is Britain's high-tech global listening post that monitors communications around the world.

Intelligence sources said both Mr Spicer and General McMaster, the US National Security Adviser, have now apologised over the claims.

Britain's ambassador to the United States Sir Kim Darroch spoke directly to Mr Spicer about the incident.

No 10 today said it had received assurances the claims would not be repeated.

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The White House latched onto a disputed Fox News report Thursday, with Sean Spicer pointing to a March 14 Fox & Friends broadcast in which Judge Andrew Napolitano claimed he had three sources telling him that Obama used Britian's GCHQ intelligence agency to spy on Trump as evidence that Trump was tapped

'The apology came direct from them,' an intelligence source told theTelegraph.

Another source told the Sun: 'Under the Five Eyes convention, we never spy on our main allies, and that includes the United States.

'This allegation is so off the scale crazy, it's very hard to understand.'

The White House latched onto a disputed Fox News report Thursday in its latest battle with the press to produce evidence of President Donald Trump's claim that Barack Obama wiretapped him.

Just before press secretary Sean Spicer was due to take questions from reporters on camera, the Republican head of the Senate Intelligence Committee and the panel's top Democrat released a joint statement affirming a lack of evidence to support the president's claim.

ABC News' Jon Karl attempted to question Spicer about their statement and similar comments that were made yesterday at a news conference by the top lawmakers on the House's Intelligence panel.

In a nearly 10-minute dressing down of Karl, Spicer pointed to a March 14 Fox & Friends broadcast in which Judge Andrew Napolitano claimed he had three sources telling him that Obama used GCHQ to spy on Trump.

In a rare public rebuke, a statement from GCHQ said: '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.'

Earlier, reading from a transcript of Fox & Friends, Spicer repeated Napolitano's claims.

'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,' he said.

Spicer continued with the citation, saying, in the words of Napolitano, 'What is that? It's the initials for the British intelligence spying agency.

'They have 24/7 access to the NSA database, so by simply having two people go to them and saying, 'President needs transcripts of conversations involving candidate President-elect Trump,' he's able to get it and there's no American fingerprints on it.'

This undated photograph shows the British Government Communications Headquarters (GCHQ) in west central England

British officials have denied Napolitano's report. 'No part of this story is true,' a British government official told Fox News. British PM Theresa May is seen above

Immediately after the broadcast a spokesperson for the British government told Fox that 'no part of this story is true.'

A British security official told Reuters the same thing. The claim was 'totally untrue and quite frankly absurd,' the representative of the British government said.

The Fox report was one of the more suspect sources of information from Spicer's list, which included reports from the New York Times and other outlets the White House routinely cites in its arguments about Trump Tower and the possibility of surveillance.

Spicer claimed later in his briefing that he was not 'casting judgement' on the veracity of the Fox report.

'I think the idea is to say that if these organizations and these individuals came to these conclusions, they merit looking into,' he said.

Since Trump initially claimed that that his predecessor was monitoring his calls, the White House has been searching for evidence to back up his claim.

Getting nowhere, Spicer changed his story last Friday and argued that the president was referring to a broad range of surveillance techniques, even though he named phone tapping as the way he was spied on.

Trump confirmed Wednesday in a Fox News interview that he was referring to surveillance of all kinds, not just the form he specified in his tweets.

Reading from a transcript of Napolitano's remarks, Spicer said, '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'

The FBI has so far refused to respond to calls from Congress to address the charges in public. It told two senators who wrote a letter asking for all information pertaining to warrants for Trump, his business, his campaign or any of his operatives that it would give them a briefing, but only in a classified setting.

This afternoon, the two most powerful members of the Senate Intelligence Committee said they have no reason to believe Trump.

'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,' Sens. Richard Burr of North Carolina and Mark Warner of Virginia said in a statement.

Burr is the committee's Republican chairman; Warner is its ranking Democrat.

Their statement comes just one day after their counterparts on the House Intelligence Committee cast similar doubt on Trump's claims.

Democratic Rep. Adam Schiff and Republican Rep. Devin Nunes, both of California, also said they have not seen any evidence to back up Trump's claims - but they hinted that their access to relevant information is by no means sufficient to make a final determination.

Napolitano had said: 'They have 24/7 access to the NSA database, so by simply having two people go to them and saying, 'President needs transcripts of conversations involving candidate President-elect Trump,' he's able to get it and there's no American fingerprints on it'

'To date I've seen no evidence,' Schiff said at a Wednesday news conference.

Nunes said, 'We don't have any evidence that that took place.'

Sen. Lindsey Graham, one of the legislators who sent a letter to the FBI asking for proof of a warrant, said in response to his Senate colleagues' statement that he strongly believes it should not be taken as a 'substitute for a public response from the Federal Bureau of Investigation and Department of Justice on this matter.'

'It is entirely acceptable for DOJ and the FBI to make a simple statement that goes to the heart of the matter – without jeopardizing classified information,' Graham said. 'I believe such a statement would serve the public well, and I fear that without an official answer this issue will continue to linger.'

The legislators' respective statements hung thick in the air in the White House Press Briefing room on Thursday and reporters waited to confront Spicer about the new development.

Trump's spokesman scolded reporters for giving statements casting doubt on the president's claims so much attention.

'When Devin Nunes comes out and says its very possible yesterday, there's crickets from you guys,' he exclaimed.

He proceeded to engage in a contentious back and forth with Karl, eventually arriving at his citation of Napolitano as a source of evidence for the president's claim that the Obama administration was monitoring his calls.

Spicer stopped reading aloud before the point in the broadcast where Napolitano claimed that the American intelligence operative working with Obama was suspiciously no longer working in government.

'What happened to the guy who ordered this? Resigned three days after Donald Trump was inaugurated,' Napolitano said.

Trump said Wednesday in a Fox News interview that he was referring to surveillance of all kinds, not just the form he specified in his tweets - wiretapping. So far, the FBI has not produced evidence that he was spied on in any way

The Fox News personality had also explained that the British government wouldn't have used a traditional wiretap.

'The concept of plugging a wire into a phonebox in the basement of a building is what the law was when the statues were written in 1978. Everything is done electronically now, via computer,' Napolitano said.

'The NSA has 24/7, 365 access to every mainframe computer of every telecom and every computer service provider that does business in the United States and they share that with various intelligence agencies including the Brits. So the British intelligence agency had this.'

British officials have denied Napolitano's core claim, nonetheless Spicer included it in his list on Thursday.

'All we're doing is literally reading off what other stations and people have reported, and I think that casts into concern, some of the activities that may have occurred during the '16 election,' he said after a reporter called the Fox report into question.

Spicer said Trump had not asked British Prime Minister Theresa May if her government spied on him.

'That happened, I think two days ago, it was something that was you know reported on air,' he said of the Tuesday Napolitano broadcast. 'It has not been raised.'