Mike Pompeo, US secretary of state, called it a “momentous decision”. Indeed such was its significance — and the pressure exerted by Washington — that two British prime ministers have been haunted by the Huawei question.
Mr Pompeo is in London this week and is expected to have a frank exchange of views with UK prime minister Boris Johnson, who finally gave the go-ahead on Tuesday for the Chinese telecoms equipment maker to supply kit for Britain’s fifth-generation mobile phone network.
The Trump administration views the decision by the UK National Security Council as a craven surrender of national sovereignty over data to the Chinese state, warning that it could give Beijing a backdoor route to spy on British communications.
Senior figures in Washington fear that other allies could now follow Britain’s lead on Huawei. Former UK defence secretary Liam Fox said Mr Johnson risked giving a “green light to other countries which don’t have the UK’s security capabilities”.
Given that Britain leaves the EU on Friday and is looking to strike a swift post-Brexit trade deal with the US, the Huawei decision raises tensions in the “special relationship” at a highly sensitive time.
The saga has been agonising to watch. While Mr Pompeo tweeted on Sunday that the UK had a “momentous decision” to take on Huawei, it has been a slow-motion process that occupied vast amounts of time for British ministers and officials.
In April last year the then prime minister Theresa May approved Huawei’s role as a supplier of “non-core” parts of the UK’s 5G network.
The company’s kit is cheap compared with rivals and security chiefs advised that any security risks could be controlled because non-core equipment does not involve the processing or storing of sensitive data.
But then Washington complained, the decision was put into a Whitehall review, and eventually — after much transatlantic rancour — Mr Johnson came to a very similar decision to Mrs May nine months later.
The facts had barely changed. Huawei is a valued supplier of antennas and base stations used in UK mobile networks, and has been for years.
However, Mr Johnson used the Conservative party’s December election manifesto to promise to roll out full fibre broadband to every household by 2025. As with mobile, Huawei has been a key supplier of equipment used in the UK fixed line networks for years.
The nine members of the National Security Council were told that banning Huawei would set back the roll out of 5G and full fibre infrastructure by up to three years, at a cost of tens of billions of pounds to a British economy struggling with low productivity.
Given that security officials at the table were happy with Huawei’s continuing role — provided it was limited to the 5G periphery and a new 35 per cent market cap was imposed — the decision at the 80-minute meeting should have been straightforward.
According to the National Cyber Security Centre, a branch of the signals intelligence agency GCHQ, the risks from using Huawei in 5G can never be removed, but they can be brought down to “acceptable levels”.
British intelligence officials believe measures agreed by the National Security Council are enough to mitigate the key threats of espionage, theft or alteration of data and network disruption. Huawei equipment will notably be completely excluded from sites such as the UK nuclear weapons base at Faslane in Scotland.
Even though Washington has repeatedly warned that a UK decision to use Huawei for 5G kit would jeopardise intelligence relationships, the day-to-day operational links between British and US spy agencies are expected to continue.
Whitehall officials highlight that intelligence-sharing is conducted independently of mainstream telecoms networks through dedicated “cryptographic” channels.
But Huawei — which has repeatedly said it is a private company and not subject to Chinese state interference — creates a neuralgia in political circles on both sides of the Atlantic.
On Tuesday British MPs were almost unanimous in condemning the continued use of Huawei. Former Conservative cabinet minister David Davis said the Chinese company should be barred, while ex-Tory leader Iain Duncan Smith described the decision was “deeply disappointing”.
The decision also caused consternation in Washington. Liz Cheney, a Republican member of Congress, tweeted: “By allowing Huawei into their 5G network, Boris Johnson has chosen the surveillance state over the special relationship.
“Tragic to see our closest ally, a nation Ronald Reagan once called ‘incandescent with courage’, turn away from our alliance and the cause of freedom.”
But Donald Trump, initially at least, refrained from tweeting about the National Security Council’s decision on Huawei.
The US president, who lobbied Mr Johnson about the Chinese company last Friday, is separately unhappy with Britain over its plan for a digital services tax that will fall heavily on US technology firms. His administration has threatened tariffs on UK car exports.
Mr Johnson has stood his ground on the digital tax, in a sign that he does not want to be presented as Mr Trump’s poodle just at the moment the UK leaves the economic security of the EU, the world’s biggest trade bloc.
While the Huawei decision could also complicate Mr Johnson’s efforts to secure a post-Brexit trade deal with the US, Mr Johnson inevitably also had one eye on China, which in recent years has seen Britain as a key destination for investment.
In 2015 the then UK chancellor George Osborne vowed that Britain would be China’s “best partner in the west”. The City of London, telecoms and even nuclear power stations were opened up to Chinese companies.
It was telling that the British government press release on “new plans to safeguard the country’s telecoms network” did not mention Huawei by name once.