The European Commission has opened a formal antitrust investigation to assess whether Google has violated European Union competition rules by favoring its own online display advertising technology services in the so-called “ad tech” supply chain, to the detriment of competing providers of advertising technology services, advertisers and online publishers.

The commission said the investigation will examine whether the U.S. tech giant is distorting competition by restricting access by third parties to user data for advertising purposes on websites and apps, while reserving such data for its own use.

“Google is present at almost all levels of the supply chain for online display advertising,” said competition commissioner Margrethe Vestager. “We are concerned that Google has made it harder for rival online advertising services to compete in the so-called ad tech stack. A level playing field is of the essence for everyone in the supply chain.”

Vestager added that the European Commission would also be looking at Google’s policies on user tracking to make sure they are in line with fair competition.