The lead-up to Britain’s formal departure from the European Union, at 11pm on Friday, has made headlines mainly due to a somewhat bathetic row over Big Ben and church bells. But in Downing Street, the fraught navigation of a post-Brexit landscape began in earnest this week, with a new kind of crisis over sovereignty.

Boris Johnson’s decision to allow the Chinese tech company Huawei a substantial role in supplying Britain’s 5G network is his first key geopolitical move, raising vital questions over data security. The green light to Huawei was given in the teeth of concerted opposition from the US and some of the prime minister’s own backbenchers. America has warned that the company’s participation in 5G networks would represent a major security risk to the west, given its close relationship to the Chinese state. Huawei has already been excluded from 5G networks in Japan and Australia on the grounds that control of vital infrastructure could fall into the hands of a potentially hostile power. One Republican senator said on Tuesday that “London has freed itself from Brussels only to cede sovereignty to Beijing”.

Such talk is overblown; sovereignty also involves Britain exercising a right to make its own decisions without transatlantic bullying. But Mr Johnson undoubtedly faces a tricky meeting with the US secretary of state, Mike Pompeo, who arrives in London on Wednesday for talks with the prime minister. Earlier this week, Mr Pompeo described the Huawei decision as “momentous” and suggested that future intelligence-sharing between Britain and America could be disrupted.

Mr Johnson has the backing of his security chiefs, who believe the risk posed by Huawei, which has operated in Britain for over a decade, is manageable. He also hopes to forestall the inevitable fallout by building in restrictions to Huawei’s future access and influence. The company has been designated a “high-risk vendor”, excluded from “core” activities of the 5G network, and its share of the market has been capped at 35%. Those limitations are sensible, but they do not eliminate risk. In Australia it was judged that the 5G era would collapse the distinction between core and periphery digital activity, making it impossible to police. In essence Mr Johnson has therefore signed up to a “known unknown”, authorising Chinese participation in vital national infrastructure where future vulnerabilities are impossible to assess. There will be a need for constant and ongoing vigilance, especially if China continues to develop its authoritarian and domineering tendencies, at home and abroad, under the leadership of Xi Jinping.

Washington has hinted that this decision may deplete stocks of goodwill ahead of post-Brexit trade negotiations that have become totemic for the government. The prime minister will hope that such talk was merely a case of playing hardball. But the decision to risk Donald Trump’s ire, at such a sensitive moment, is revealing. The power of tech-driven growth and to propel Britain to the sunlit uplands is a prime article of faith in Mr Johnson’s inner circle. Taking Huawei out of the 5G equation would have significantly delayed the introduction of high-speed internet across the country. That was a price the prime minister and his advisers were not willing to pay, even if that meant queering the post-Brexit pitch in trade terms. Mr Johnson has taken a gamble. On balance, given the advice of his own intelligence services, it was justifiable. But it is not, on any front, risk-free.