British regulators are to investigate whether Facebook broke data protection laws by letting researchers conduct a psychological experiment on unknowing users.
The Information Commissioner's Office said it would be asking the social network questions and would also liaise with Irish regulators, as the company's European headquarters are in Dublin.
Although the regulator has not specified which laws the site may have infringed, it has the power to force companies to change their policies and can also issue fines up to £500,000 for breaching the Data Protection Act.
More top news
David Cameron sets out his case for British jets and drones to strike so-called Islamic State targets in Syria in an address to the Commons.
A drink-driver who "destroyed the lives" of two mothers was allowed to have his curfew lifted so he could go on a foreign stag party.
Demonstrators have blockaded the entrance tunnel to Heathrow Airport in apparent protest at plans to build a third runway.