LONDON -- European Union regulators have launched a fresh antitrust investigation of Google, this time over whether the U.S. tech giant is stifling competition in digital advertising technology.

       The European Commission said Tuesday that it has opened a formal investigation into whether Google violated the bloc's competition rules by favoring its own online display advertising technology services at the expense of rival publishers, advertisers and advertising technology services.

       The commission, the EU’s executive arm and the bloc’s top antitrust enforcer, is looking in particular at whether Google is restricting access by third parties to user data for ad purposes on websites and apps.

       “Online advertising services are at the heart of how Google and publishers monetize their online services,” said Margrethe Vestager, the EU commission's competition chief and executive vice president for digital. Google collects data to be used for targeted advertising while it also sells advertising space and acts as a middleman between online advertisers and publishers, she said.

       “We are concerned that Google has made it harder for rival online advertising services to compete in the so-called ad tech stack,” Vestager said.

       An email seeking comment was sent to Google's press office.