he test saw Facebook "manipulate" the news feeds of nearly 700,000 users to control which emotional expressions they were exposed to.

The Information Commissioner's Office (ICO) said it planned to question Facebook over the study.

Facebook said it had taken "appropriate protections for people's information".

"We are happy to answer any questions regulators may have," Facebook's Richard Allen said in a statement.

The Financial Times and The Register quoted the ICO as saying that it would contact Ireland's data protection regulator over the issue.

Facebook's European headquarters are based in Dublin.

The research was conducted in collaboration with Cornell University and the University of California at San Francisco on 689,000 Facebook users over a period of one week in 2012.

According to the report on the study: "The experiment manipulated the extent to which people were exposed to emotional expressions in their News Feed".

The study found that users who had fewer negative stories in their news feed were less likely to write a negative post, and vice versa.

The research was done to gauge if "exposure to emotions led people to change their own posting behaviours".

However, the research has been criticised because Facebook users were unaware they were taking part.

Labour MP Jim Sheridan, a member of the Commons media select committee, has called for an investigation into the matter.