Rivals of WhatsApp have reportedly been approached by European Union antitrust officials regarding the proposed $19 billion acquisition of the world’s most popular messaging app maker by Facebook.

Sources told The Wall Street Journal that detailed questionnaires have been sent to a number of technology and messaging companies, with the aim of gauging how the deal could potentially affect competition in the market.

At least two of the companies are believed to have expressed concern that the deal would squeeze them out of the market, due to the combined online messaging volume of Facebook and WhatsApp.

The companies have also been asked about how they control and use personal data within the services they offer.

The move comes ahead of a formal review of the deal by the European Commission, which is unlikely to have taken place had Facebook not requested it, as WhatsApp doesn’t generate enough revenue in Europe to trigger an investigation automatically.

The social networking giant asked the EC to conduct a review covering all 28 countries within the EU in May to avoid the possibility of separate reviews in multiple markets.

According to The Wall Street Journal, the telecoms industry in Europe has been lobbying against the merger on the grounds that OTT companies are using their infrastructure but aren’t taxed or regulated in the same way.

However, some antitrust lawyers believe the argument could be difficult to justify due to the fast-changing nature of online communications and how broadly the market is defined.

The US Federal Trade Commission notified Facebook and WhatsApp about their obligations to protect consumer privacy in April, including the fact that post-merger WhatsApp must continue to honour prior promises made to consumers.