Though Dr Kanga said iOmniscient had no "ongoing relationship as such" with the Hong Kong police, he said it was likely that - as with the majority of his clients - they would be on a maintenance contract, granting them access to annual or biannual software updates.

"If you [sold] somebody a birthday cake, you don't know what they've done with it," he said, before adding that a better analogy might be selling someone a cake knife that later gets used to "kill someone else".

The Hong Kong government has not commented on whether it is using facial recognition technology to police protests. But its decision earlier this month to invoke emergency laws banning face masks sparked fears about the use of advanced surveillance to target demonstrators.

iOmniscient's technology conformed to the European Union's data protection standards, which meant that clients usually could not access the faces stored in their own system. But in the case of an emergency such as a missing child, the client could reverse the redaction for selected individuals.

University of Queensland professor Brian Lovell, an expert in facial recognition who sold some of his technology to iOmniscient in 2011 but has no relationship with the company, said it was not unusual that a software developer would be unable to monitor the use of their product after the point of sale.

"You’d be pretty annoyed if Bill Gates can read all your Word documents," he said.

Professor Lovell acknowledged, however, that advances in convolutional neural networks (a form of artificial intelligence) had led to vast improvements in facial recognition, and that companies needed to take precautions such as limiting the number of cameras linked to their software.

He also noted that traditional weapons systems such as those for missiles tended to have a "kill switch" that enabled manufacturers to shut down their products in emergency situations, but this was uncommon for software and its introduction would likely be met with legal challenges.

Dr Kanga said his business was "politically neutral" but drew some moral lines. Omniscient had not sold facial recognition technology to clients in mainland China, and would not do business with regimes such as those in North Korea or Iran.

Asked about the unrest in Hong Kong, he was diplomatic: "I would hope that in the long term [China] can maintain the 'one China, two systems' policy, which I think is Hong Kong's strength."