Facebook has come under fire for quietly expanding the availability of technology to automatically identify people in photos, renewing concerns about its privacy practices.

The feature, which the giant social network automatically enabled for its more than 500 million users, has been expanded from the US to “most countries”, Facebook said on its official blog on Tuesday.

Marc Rotenberg, president of the non-profit privacy advocacy group Electronic Privacy Information Center, said the 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 its 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.

Internet security consultancy Sophos noted that many Facebook users had seen the facial recognition option turned on without any notice in the last few days.

“Yet again, it feels like Facebook is eroding the online privacy of its users by stealth,” commented Graham Cluley, a senior technology consultant at Sophos.

Facebook’s “Tag Suggestions” feature uses facial recognition technology to speed up the process of labeling friends and acquaintances in photos posted on the site.

Facebook has been repeatedly criticised for changing settings involving privacy and identity in favour of making more data public in ways that means its users have to opt out of, rather than opt in to, the service.

Facebook, which announced in December that it planned to introduce the facial recognition service in the US, acknowledged that the feature was now more widely available.

The site also 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.

While other photo software and online services such as Google Inc’s Picasa and Apple Inc’s iPhoto use facial recognition technology, its use on a social network like Facebook could raise thorny privacy issues.

Google has stepped away from the widespread implementation of its Google Goggles service, which would try to identify people based on facial recognition through mobile phones running its Android operating system. Instead it only uses it for translating text and identifying objects. Eric Schmidt, Google’s chairman, said earlier in June that he had concerns about its use with people.

“We do have the relevant facial recognition technology at our disposal. But we haven’t implemented this on Google Goggles because we want to consider the privacy implications and how this feature might be added responsibly,” he said. “I’m very concerned personally about the union of mobile tracking and face recognition.”

Rotenberg noted that Apple’s iPhoto software gave users control over facial recognition technology by letting them elect whether or not to use it with their personal photo collections.

Facebook’s technology, by contrast, operates independently, analysing faces across a broad swathe of newly uploaded photos.

Last year the Electronic Privacy Information Center filed a complaint about Facebook’s privacy practices with the US Federal Trade Commission, which Rotenberg said was still pending.

He noted that he planned to take a close look at Facebook’s new announcement involving facial recognition technology.