The US has agreed not to repeat claims the UK’s communications intelligence agency wiretapped Donald Trump in the weeks after he won the US election.
GCHQ denied allegations made by the White House that it spied on Mr Trump as president-elect.
No 10 has been assured the allegations would not be repeated, a spokesman for Prime Minister Theresa May said.
He said it had been made clear to US authorities the claims were “ridiculous and should have been ignored”.
GCHQ also rejected the allegations as “nonsense” and “utterly ridiculous”. The unusual move by the agency came after White House press secretary Sean Spicer cited claims first made on US TV channel Fox News earlier this week.
Mr Trump has claimed that Trump Tower in New York was under surveillance.
But he has provided no evidence for the claim.
A Senate committee on Thursday concluded that there were “no indications” Trump Tower was under surveillance by the US government before or after the election.
The claims of GCHQ involvement were initially made by former judge Andrew Napolitano.
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Mr Spicer quoted Mr Napolitano as saying: “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.”
A GCHQ spokesman said: “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.”
GCHQ is one of three UK intelligence and security agencies, along with MI5 and MI6. It delivers signals intelligence – the intercepting of communication between people or electronic devices – to the prime minister and foreign secretary.
The agency credits its “particularly strong” relationship with its US equivalent, the National Security Agency, to the collaboration it began at Bletchley Park during World War Two.
During the war, Bletchley Park housed the Government Code and Cypher School (GC&CS) which worked on cracking the military codes that secured German, Japanese and other enemy nation’s communications.
By Gordon Corera, BBC security correspondent, in Washington
It is unusual for GCHQ to comment directly on a report about its intelligence work, normally preferring to stick to the policy of neither confirming nor denying any activity.
The phrase “utterly ridiculous” is also very unusual for the agency.
But it’s a sign of just how seriously they take it. The allegations are so sensitive that the agency clearly felt they could not let them go unchallenged.
Donald Trump’s claim that the Obama administration had ordered surveillance on him has generated enormous attention but with so far no evidence to back it up.
And British intelligence is clearly determined to avoid being drawn into what is an increasingly toxic row in Washington.
On Thursday Senate Intelligence Committee chairman Richard Burr dismissed Mr Trump’s claim his phones were tapped.
Mr Trump had accused his predecessor Barack Obama of wiretapping Trump Tower during the presidential race.
Mr Spicer said Mr Trump stuck by his allegations.
US will ‘not repeat’ claims GCHQ wiretapped Donald Trump}