Facebook has rolled out facial recognition technology which monitors people's photos in the UK, in a move which has made a security expert question the privacy implications of the service.
The social networking site is now searching its servers for photos that look like UK Facebook users, and asking friends of those users to tag the photos, according to Sophos senior technology consultant Graham Cluley.
"Facebook not only gathers information about what you do, but also what you look like," Cluley told ZDNet UK on Tuesday.
The service was rolled out in North America in December 2010. Facebook had not provided information about when photo recognition was rolled out to the UK at the time of writing.
Cluley said in a blog post: "The tagging is still done by your friends, not by Facebook, but rather creepily Facebook is now pushing your friends to go ahead and tag you."
Users have found that Facebook had enabled photo recognition outside of the US in the last few days, said Cluley, who criticised the company for enabling photo recognition by default.
"Unfortunately, once again, Facebook has added a new feature to share additional information about its users, and turned it on by default," said Cluley. "Most Facebook users still don't know how to set their privacy options safely, finding the whole system confusing. It's even harder though to keep control when Facebook changes the settings without your knowledge."
Facebook uses an algorithm to compare new photos with people in old photos, and suggest tags.
A Facebook spokeswoman told ZDNet UK on Tuesday that automatic tagging was "just making the whole process easier".
"Only your friends can tag you in photos so nothing has changed to the way photo tagging works," said the spokeswoman. "Previously, you had to tag each photo individually which could be very time-consuming."
Facebook founder Mark Zuckerberg acknowledged in May that users were worried about Facebook privacy when new services were rolled out, but said that people get used to the services.
Update Wednesday 8-6-11: Facebook confirmed on Tuesday that it had been rolling out facial recognition on a rolling basis:
"We launched Tag Suggestions to help people add tags of their friends in photos; something that's currently done more than 100 million times a day. Tag Suggestions are only made to people when they add new photos to the site, and only friends are suggested. If for any reason someone doesn't want their name to be suggested, they can disable the feature in their privacy settings.
"When we announced this feature last December, we explained that we would test it, listen to feedback and iterate before rolling it out more broadly. We should have been more clear with people during the roll-out process when this became available to them. Tag Suggestions are now available in most countries and we'll post further updates to our blog over time."
Facebook said in a updated blog post that it had been rolling-out facial recognition "over the past several months".