FRANKFURT (Reuters) - The vast troves of consumer data held by big Internet companies should be scrutinized in merger probes because they have a big impact on competition, the president of the German antitrust watchdog told a newspaper.

An illustration photo shows Whatsapp App logos on a mobile phone in Sao Paulo, Brazil, December 16, 2015. REUTERS/Nacho Doce

“Until now, markets in which no money flows and in which no revenues are posted do not count as markets from a competition point of view. But that obviously goes against the logic of many Internet markets,” Andreas Mundt told Sueddeutsche Zeitung’s Saturday edition.

Collections of “big data” - covering billions of internet searches, messages and other online interactions - hand Internet companies huge power they can exercise in marketing and commerce and which potentially makes it difficult for smaller businesses to compete in those areas.

Facebook's FB.N $19 billion acquisition of Whatsapp in 2014 almost escaped scrutiny from cartel authorities as the messaging service hardly had any revenues at the time, Mundt said, adding that it was apparently still of great strategic importance to Facebook.

“How many users are there and which data is concerned? Those are the better measurement categories when it comes to defining competition in the Internet,” he said, adding that lawmakers should clarify rules.

While the German cartel authority does take data issues into account when making decisions, these may be successfully challenged in court due to the lack of clear legislation, he added.

The German competition watchdog has set up a six-person Internet task force which is working on drafting plans on how German law can be adapted to the Internet age, he said.

Mundt’s comments come after the European Union also started to take a harder look at whether the use of “big data” by Internet companies violates competition rules.

Since taking over as Europe’s top antitrust enforcer in 2014, Margrethe Vestager has stepped up investigations into U.S. web giants such as Google and Amazon to decide whether her agency should regulate them more tightly.

"If just a few companies control the data you need to satisfy customers and cut costs, then you can give them the power to just drive rivals out of the market," Vestager said last month. Last April, the European Commission accused Alphabet Inc's GOOGL.O Google of favoring its own shopping services in search results at the expense of rivals, and is weighing possible sanctions against the world's most popular search engine. Previously, the EU considered and rejected big data concerns when it approved Google's acquisition of online advertising firm DoubleClick in 2008 and Facebook's purchase of WhatsApp.