Computer software which can identify unnamed faces and then collate photographs of that person is to be released to the public amid concern among privacy campaigners.
The programme works by scanning the relative positions of the eyes, nose and mouth and claims to be accurate in nine out of 10 cases.
It can then search the internet for further images of the same individual and, in tests, unearthed untagged photos which had not previously been seen by some of the people in them.
The managers of Face.com, which created the software, told the Sunday Times that 5,000 developers were already using it. Gil Hirsch, its chief executive officer, said: "You can basically search for people in any photo. You could search for family members on Flickr, in newspapers, or in videos on YouTube." Such software has previously been the domain of Governmental organisations such as the UK Border Agency or inside social networking platforms.
The Information Commissioner's Office said there were no legal restrictions on the use of facial recognition software. However, Simon Davies, of Privacy International, said: "The regulators have been hugely behind the curve on protecting people's privacy on the internet. We need to push for much tighter international rules."