Facebook is facing a potential fine after Europe's anti-trust watchdog accused it of providing "incorrect or misleading" information regarding its 19 billion US dollar (£15.3 billion) takeover of WhatsApp.
In a statement of objections sent to the US social network, the European Commission said that during a 2014 probe into the deal Facebook inaccurately claimed that it would be unable to combine user data between the two companies' accounts.
The commission said the "technical possibility of automatically matching Facebook users' IDs with WhatsApp users' IDs already existed in 2014".
Commissioner Margrethe Vestager said: "Companies are obliged to give the commission accurate information during merger investigations. They must take this obligation seriously.
"Our timely and effective review of mergers depends on the accuracy of the information provided by the companies involved. In this specific case, the commission's preliminary view is that Facebook gave us incorrect or misleading information during the investigation into its acquisition of WhatsApp. Facebook now has the opportunity to respond."
Facebook has until January 31 to respond to the statement of objections. If the commission's concerns are confirmed, it could impose a fine of up to 1% of Facebook's turnover.
News of a potential sanction comes as relations between the EU and US tech giants are strained.
Ms Vestager has recently put the tax affairs of a number of high-profile firms including Apple, Amazon and Google under the microscope.
The commission said the new investigation does not affect its decision to approve the merger.
In an initial statement, a Facebook spokeswoman said: "We respect the commission's process and are confident that a full review of the facts will confirm Facebook has acted in good faith.
"We've consistently provided accurate information about our technical capabilities and plans, including in submissions about the WhatsApp acquisition and in voluntary briefings before WhatsApp's privacy policy update this year.
"We're pleased that the commission stands by its clearance decision, and we will continue to co-operate and share information officials need to resolve their questions."
Facebook's turnover in 2015 was more than £14.5 billion.