The White House has promised allegations that GCHQ spied on Donald Trump will "not be repeated", according to the Prime Minister's official spokesman.

It follows an angry denial from Britain's intelligence agency - not normally known for confirming or denying any of its activity - that it had helped former president Barack Obama spy on Mr Trump before his election to the presidency.

Speaking on Friday, Theresa May's spokesman said the government had "made clear" to the US that the "ridiculous" claims should be ignored, and had received assurances in return that they will not be repeated.

That, he added, showed the administration in Washington did not lend them any credence.

The claims were made by White House press secretary Sean Spicer, who repeated an unsubstantiated allegation from Fox News analyst Andrew Napolitano.

"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," the former New Jersey judge had told the programme.

Mr Spicer later quoted Mr Napolitano as he tried to justify the president's repeated - but equally unsubstantiated - insistence that his predecessor put Trump Tower under surveillance during the campaign.