US warns Boris Johnson it would be 'madness' to give Huawei a role in Britain's 5G network

American officials on Monday mounted a last-ditch effort to persuade the UK not to allow the Chinese telecoms giant to play any part in Britain's mobile infrastructure amid security fears.

Donald Trump's deputy national security adviser, Matt Pottinger, handed British ministers a dossier on the alleged threat posed by the firm ahead of a crunch decision by the Prime Minister.

The government has already indicated that Huawei - which faces an outright ban in US infrastructure decisions - will be allowed to run 'non-core' parts of Britain's high-speed mobile network, including supplying mobile phone masts and antennas.

A final decision on whether to press ahead is expected soon.

But a US official told The Guardian: "Donald Trump is watching closely."

And one official told The Times: "Any amount of [Huawei] equipment carries risk of compromising very sensitive infrastructure, carries the risk of compromising private citizens’ data, as well as corporate secrets and other sensitive information. It raises the question — why do any amount of business with someone that you don’t trust?"

Huawei has consistently denied allegations that it is influenced by the Chinese state or poses any security risk.

A spokesperson for the company said: "We are a private company which has supplied 3G, 4G and broadband equipment to the UK's telecoms companies for 15 years. British experts are clear our technology does not pose a security risk."

Moves to hand Huawei a role in the UK's 5G infrastructure sparked a major Cabinet row last year, while Conservative MP Bob Seely has urged Parliament's Foreign Affairs Committee to open an investigation into the firm.

The preliminary decision to press ahead with allowing Huawei a role in Britain's 5G network came at a meeting of the National Security Council, which voted 5 to 4 in favour of the move.

Details of the highly confidential meeting were subsequently leaked to the press, with a Whitehall probe pointing the finger at the then-defence secretary Gavin Williamson, who was then sacked by Theresa May. 

A government spokesperson said: "The security and resilience of the UK's telecoms networks is of paramount importance.

"The government continues to consider its position on high risk vendors and a decision will be made in due course."