The use of the technology in Britain has long been criticised by civil liberties groups such as Amnesty International, and faces a blanket ban in the European Union.

Biofire's gun can also be enabled by a fingerprint reader, one of several smart gun features designed to avoid accidental shootings by children, reduce suicides, protect police from gun grabs, or render lost and stolen guns useless.

The plans are described as safe city projects aimed at maintaining security and, in some cases, preserving civil peace, say the people who are or have been involved in the projects.

Meta Platforms Inc says more than one-third of daily active users have opted into the face recognition setting, and the change will now delete the 'facial recognition templates' of more than 1 billion people.

Similar systems are already in place in some Chinese cities.