Security firm Sophos has accused Facebook of "eroding the online privacy of its users by stealth" after it rolled on facial recognition technology without alerting social networkers.

The technology, which was first rolled out in the US last year, uses face recognition software to look through your current photos and will match people in new photos you upload to those who you have already tagged.

The technology is automatic so users must 'opt-out' via their privacy settings. However, while Facebook alerted US users weeks ahead of the technology being made available, the roll-out has not been flagged up to European and other users., Instead, the European social networkers did not have the new service flagged. Instead, many have just discovered the feature is already running.

"Unfortunately, once again, Facebook seems to be sharing personal information by default. Many people feel distinctly uncomfortable about a site like Facebook learning what they look like, and using that information without their permission," said Graham Cluley from Sophos in a blog.

"If Facebook begins to learn what you look like how are they going to use that information in the future? Are they going to share that data with other sites? People find it a little bit creepy and it makes their skin crawl a bit."

Facebook admitted it's been rolling out Tag Suggestions over the last several months and the feature "is now available in most countries".