A CCTV watchdog has written to police chiefs to warn that the increased use of facial recognition technology risks infringing people's privacy.
The software is used by forces to keep watch of large crowds such as at the Notting Hill Carnival, and track down suspects including in terror cases.
The Government's independent Surveillance Camera Commissioner Tony Porter wrote to the National Police Chiefs Council, the Financial Times reported, noting the "significantly increased capabilities to intrude upon the privacy of citizens".
In the letter he added that “if not responsibly considered and regulated” it could undermine public confidence.
In an interview with the FT, Mr Porter said more transparency was needed on how the police image database is used in combination with the technology, which is being introduced in an unknown number of forces.
“Are the forces aware of the regulatory requirements in using this technology?” he said.
“We don’t have transparency," he added, adding that it is not known whether the techniques being used "are proportionate and legitimate.”
The comments come amid growing unease with the level of use of mugshots held by police in combination with facial recognition technology.
Back in September, Biometrics Commissioner Paul Wiles revealed that 19 million custody photographs were on file on the Police National Database - but many were of people who have since been released without charge.
Professor Wiles said technological developments had outpaced legislation, and warned the situation threatened to damage the reputation of the police.
He told BBC Radio 4's Today programme: "I think it's very worrying because if we're not careful the public will lose confidence in the police."
At present, people have the right to request the removal of their custody images if they have been cleared.
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