The claim that GCHQ carried out surveillance on Donald Trump during the election campaign is "arrant nonsense", Rick Ledgett, the number two at the US National Security Agency (NSA) has told the BBC in an exclusive interview.
A commentator on Fox News had claimed that GCHQ had carried out the activity on America's behalf, but Mr Ledgett said the claim showed "a complete lack of understanding in how the relationship works".
"Of course they wouldn't do it. It would be epically stupid," he told me.
"Our job in the intelligence community is to be apolitical. Our job is to speak truth to power," he emphasised.
The origins of much of the tension lie in the assessment by the US intelligence community that Russia interfered in the presidential election, and the subsequent reaction from Donald Trump.
Mr Ledgett said the evidence of Russian involvement was "extraordinarily strong" and "irrefutable" and that the NSA had played a key role in establishing the case.
Mr Ledgett said he was "dead solid 100% confident" that the Russian state was behind the attempts - although he said it was not for the intelligence community to evaluate the actual impact of those attempts on the vote itself.