Asked if President Trump making "baseless" claims against British security services damaged the close relationship between the two nations, the NSA director said: "I think it clearly frustrates a key ally of ours".
Meanwhile, the directors of the Federal Bureau of Investigation and National Security Agency were due to speak publicly today about the claims of alleged links between Russian Federation and president Trump's campaign.
Comey was testifying Monday (March 20) before the House intelligence committee.
"This is a risky game", Westmacott warned in an opinion piece in The Guardian.More news: Arsenal boss Arsene Wenger downplays financial importance of UEFA Champions League qualification
He warned it risked undermining a relationship that was "critical" to US-UK efforts to counter a range of threats including terrorism, Russian aggression, Chinese cyber-attacks, and the nuclear threat from North Korea.
Westmacott, who was the UK's ambassador to Washington from 2012 to 2016, says "gratuitously damaging [the relationship] by peddling falsehoods and then doing nothing to set the record straight would be a gift to our enemies they could only dream of". "Second, the idea of the British foreign secretary signing a warrant authorizing such an intrusion into domestic United States politics was unthinkable".
Westmacott also wrote disdainfully about Trump's "famous reluctance to admit mistakes".
Earlier this month, White House Press Secretary Sean Spicer referred to unsubstantiated allegations made by a Fox News analyst that GCHQ, the British electronic intelligence agency, had helped Obama wiretap Trump.More news: Undefeated Israel beats Netherlands in World Baseball Classic
The claim brought a rare public denial by GCHQ which described the claims as "utterly ridiculous" while Downing Street said it had secured an assurance that the allegation would not be repeated.
When Trump was questioned about the "wiretapping" saga during a meeting with German chancellor Angela Merkel on Friday, Trump said Spicer was simply repeating a media report, the Huffington Post reports.
Nunes says he hopes the committee's hearings will result in a "definitive report" on Russia's involvement in the presidential election.
"That includes investigating the nature of any links between individuals associated with the Trump campaign and the Russian government and whether there was any coordination between the campaign and Russia's efforts", Comey said in his opening statement to the committee.More news: Arsene Wenger 'Determined' to Stay at Arsenal
Fox News has disassociated itself from the allegations, saying there is no evidence to back them.
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