Facebook has eroded the online privacy of millions of its users by switching on facial recognition technology without telling them, a technology expert said today. The leading social networking website has \'enabled\' a function that  automatically identifies people in photos without their knowledge. The feature has been expanded from the United States to \'most countries\', Facebook said on its official blog yesterday. Its \'Tag Suggestions\' feature uses facial recognition technology to speed up the process of labeling friends and acquaintances that appear in photos posted on Facebook. So if a friend \'tags\' you in one photo, the technology will automatically scan your face and then try and find matches among all their pictures. It will then suggest that they \'tag\' these photos of you as well. Tagged photos typically appear in your photo stream as well as in your friends streams, depending on your settings. When the function was launched in the U.S, Facebook contacted users directly a few weeks before - an action not taken in Europe. Internet security consultant firm Sophos first reported the change yesterday, after Facebook users reported that the site had enabled the facial recognition option in the last few days without giving users any notice. \'Yet again, it feels like Facebook is eroding the online privacy of its users by stealth,\' wrote Graham Cluley, a senior technology consultant at Sophos. Facebook, which announced in December that it planned to introduce the service in the United States, acknowledged on Tuesday that the feature was in fact now more widely available. When asked about the Sophos blog post, Facebook said in an emailed statement that \'we should have been more clear with people during the roll-out process when this became available to them\'. The statement noted that the photo-tagging suggestions are only made when new photos are added to Facebook, that only friends are suggested and that users can disable the feature in their privacy settings. The company did not respond to requests for further comment. While other photo software and online services such as Google Inc\'s Picasa and Apple Inc\'s iPhoto use facial recognition technology, the use of the technology on an Internet social network like Facebook, which counts more than 500million users, could raise thorny privacy issues. Marc Rotenberg, President of the non-profit privacy advocacy group Electronic Privacy Information Center, noted that Apple\'s iPhoto software gave users control over facial recognition technology by letting them elect whether or not to use the technology with their personal photo collections. Facebook\'s technology, by contrast, operates independently, analysing faces across a broad swathe of newly uploaded photos. Mr Rotenberg said such a system raised questions about which personally identifiable information, such as email addresses, would become associated with the photos in Facebook\'s database. He also criticised Facebook\'s decision to automatically enable the facial-recognition technology for Facebook users. \'I\'m not sure that\'s the setting that people would want to choose. A better option would be to let people opt-in,\' he said. A spokeswoman from Facebook said: \'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.\' Last year the Electronic Privacy Information Center filed a complaint about Facebook\'s privacy practices with the U.S. Federal Trade Commission, which Mr Rotenberg said was still pending.