Facebook has come under fire for switching on facial recognition software without telling its users.

The technology automatically identifies people in photos uploaded to the site, and encourages friends to tag them.

Facebook has rolled out Tag Suggestions to “most” of the social network’s 600 million users worldwide after first testing it in the US.

Company bosses have admitted they “should have been more clear” about the expansion of the software.

“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,” a Facebook spokesman told The Register.

“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.”

The response was only offered after Facebook was quizzed about why the facial recognition technology was applied to accounts without holders’ knowledge, rolling it out as default rather than allowing users to ‘opt in’.

“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,” said the Facebook spokeswoman.

“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,” she added.

Graham Cluley, of the British internet security firm Sophos, told The Telegraph the site’s actions had once again engulfed it in a row over privacy.

“Yet again, it feels like Facebook is eroding the online privacy of its users by stealth,” he said.

“Many people feel distinctly uncomfortable about a site like Facebook learning what they look like, and using that information without their permission.”