UK privacy advocacy group Big Brother Watch filed a complaint with the country's privacy watchdog over the facial recognition search engine PimEyes. 

In a formal complaint (pdf) to the UK's Information Commissioner's Office (ICO), Big Brother Watch accused PimEyes of unlawfully processing the biometric data of millions of UK citizens, arguing it failed to obtain permission from those whose images had been analysed. 

It went to say that PimEyes enabled 'surveillance and stalking on a scale previously unimaginable' by making it easy for users to identify where an individual worked or lived. 

The tool, it said, could easily be used by potential employers, university admissions officers, domestic abusers or stalkers, and could threaten 'end anonymity as we know it'.

PimEyes CEO Giorgi Gobronidze responded by saying the service posed fewer stalking risks than social media services.