Visitors to “the Cell” have their phones confiscated at reception. Taking pictures is forbidden and staff working inside this nondescript office block on the outskirts of Banbury in Oxfordshire have been subjected to “developed vetting” — the most stringent form of government security clearance required for those with access to top-secret material.

Surprisingly, these are not government employees but are on the payroll of the Chinese telecoms giant Huawei. Their job, overseen by GCHQ, is to examine Huawei’s hardware and software for any bugs or vulnerabilities that could be used for nefarious purposes.

The existence of the Cell — the nickname for the Huawei Cyber Security Evaluation Centre (HCSEC) set up in 2010 — is testament to Britain’s uneasy relationship with the world’s biggest manufacturer