Companies, which need approvals for mergers from several national regulators in Europe, typically prefer to take their case to the European Commission to avoid the hassle of multiple reviews.

The deal was cleared in the United States last month by the Federal Trade Commission (FTC).

The FTC's Bureau of Consumer Protection, in a letter to the two companies on April 10, said WhatsApp must adhere to its current privacy practices after the merger, including a promise not to use WhatsApp users' personal data for targeted ads.

Facebook was not immediately available for comment, while a European Commission spokeswoman declined to comment.