File on 4 has been tracking the roll-out of facial recognition tech across Britain’s streets, shopping centres and football grounds. The Metropolitan Police has announced it will use live facial recognition cameras operationally for the first time on London streets. The force sees the technology as a vital tool in the fight against crime. But privacy campaigners say it's a 'serious threat to civil liberties.' The pace is frenetic – new computer systems can watch thousands of people at once, with the most powerful able to operate at distances of over a mile. They can do all of this in “real-time”, meaning everyone who passes by the camera can be scanned against a “watchlist” of suspects. But technology like this means more and more innocent people are affected. Yet the public are not always explicitly warned, and neither are the regulators. File on 4 has been given new details of a trial at Meadowhall shopping centre in South Yorkshire in which police and retailers worked together to scan millions of shoppers, looking out for three suspects and a missing person (the latter was found as a result). It was one of several trials conducted by police and private companies, which went ahead despite requests from the Surveillance Camera Commissioner for police to ask him before implementing such schemes. The legislation surrounding facial recognition is new and mostly untested, leading to calls for stricter, more specific laws to be passed. Meantime, the Surveillance Camera Commissioner has called for a regime of inspections of the technology for both public and private bodies; a call backed by the veteran Conservative MP David Davis. Facial recognition may be new, but it still begs an urgent answer to an age-old question: who watches the watchers? Reporter: Geoff White Producer: Helen Clifton Editor: Carl Johnston