The White House has assured Number 10 it will not repeat claims that GCHQ spied on Donald Trump, Theresa May's spokesman has said.
He said allegations that British spies had helped Barack Obama eavesdrop on Mr Trump during the presidential campaign - cited by the White House press secretary - were "ridiculous".
"We've made clear to the administration that these claims are ridiculous and they should be ignored and we've received assurances that these allegations will not be repeated," the spokesman told reporters.
"We have a close special relationship with the White House and that allows us to raise concerns as and when they arise as was true in this case."
The spokesman added that it would be impossible for GCHQ to spy on Mr Trump as both countries are members of the Five Eyes alliance - a joint intelligence co-operation agreement which also includes Australia, Canada and New Zealand.
"I would add, just as a matter of fact, with the Five Eyes pact, we cannot use each other's capabilities to circumvent laws," he said.
"It's a situation that simply wouldn't arise."
Previously, GCHQ had also dismissed the claim as "utterly ridiculous".
The original accusation was made by US media commentator Judge Andrew Napolitano, and was then repeated by White House press secretary Sean Spicer.
Mr Spicer spoke out after the US Congress rejected Mr Trump's claim that the Obama administration "tapped his wires" before the election.
He said the President "stands by" his claim, but no evidence has yet been produced to back it up.
Mr Spicer identified a report on Fox News as the source of the allegation, saying that three intelligence sources had informed the broadcaster that Mr 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," said Mr Spicer.
The British intelligence organisation took the unusual step of responding by saying: "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."
Dominic Grieve MP, chairman of the UK's Intelligence and Security committee, said GCHQ's denial was "an unusual step by the agency, but it clearly indicates the strength of feeling about this issue".