Did Facebook News Feed Experiment Break Law?
A psychological study carried out by Facebook on its users without their consent is being investigated by the UK data watchdog.
The Information Commissioner's Office (ICO) will question the social network to establish whether strict data protection laws were broken.
If laws were contravened, the site could face a large fine.
The ICO said it will also contact the Irish data protection body because Facebook has its European headquarters in Dublin.
A spokesman told Sky News: "We're aware of this issue, and will be speaking to Facebook, as well as liaising with the Irish data protection authority, to learn more about the circumstances."
The test saw Facebook manipulate the news feeds of around 700,000 users, to see how restricting the visibility of happy or sad updates from friends affected their behaviour on the site.
But Facebook's Richard Allen said the site had implemented "appropriate protections" for people's information, adding: "We are happy to answer any questions regulators may have."
The ICO spokesman said it was too early to tell which part of the law Facebook may have breached.
The regulator has the power to fine organisations up to £500,000 and force them to change their policies if personal data is misused or processed without consent.
Facebook's week-long study took place in 2012 and found that if users were shown fewer positive stories they were more likely to post negative updates, and vice versa.
Mr Allen added: "It's clear that people were upset by this study and we take responsibility for it.
"We want to do better in the future and are improving our process based on this feedback."
Adam Kramer, a researcher on the project, said the research benefits of the study may not have justified "all of this anxiety".
He said: "The goal of all our research at Facebook is to learn how to provide a better service.
"Having written and designed this experiment myself, I can tell you that our goal was never to upset anyone."