The European Commission has opened a formal antitrust investigation into Google over whether the search giant unfairly favours its own online display advertising technology over competitors, the EU announced. The investigation will also explore whether Google is unfairly limiting access to user data to its competitors.

It’s an important investigation because it covers Google’s core online advertising business, which Reuters notes generated $147 billion in revenue for the company last year. Bloomberg reports it’s the first time the EU has investigated Google’s online display advertising business, where it serves as an intermediary between advertisers and publishers to fill ad space on web pages and apps.

“Google collects data to be used for targeted advertising purposes, it sells advertising space and also acts as an online advertising intermediary. So Google is present at almost all levels of the supply chain for online display advertising,” the European Commission’s competition chief Margrethe Vestager said in a statement. “We are concerned that Google has made it harder for rival online advertising services to compete in the so-called ad tech stack.”

“We will also be looking at Google’s policies on user tracking to make sure they are in line with fair competition,” Vestager said.

The European Commission says it’s exploring several of the company’s advertising practices, like requiring advertisers to use Google’s own Ad Manager to display ads on YouTube, and allegedly favouring its own ad exchanges. The investigation will also touch upon Google’s plans to phase out third-party cookies in Chrome as part of its Privacy Sandbox plans, as well as the upcoming changes to advertising IDs on Android.

The formal investigation comes as Google is facing similar antitrust scrutiny in the US. Last October the US Justice Department filed antitrust charges against the company, alleging it illegally monopolizes the search and ad markets.

The investigation is the latest antitrust action the EU has taken against Google. Previous investigations have covered Google’s online shopping services, its Android policies, and its AdSense contracts.

Google said it would “engage constructively” with the European Commission. “Thousands of European businesses use our advertising products to reach new customers and fund their websites every single day,” it said in a statement. “They choose them because they’re competitive and effective.” Google says many of the top advertisers use upwards of four platforms to buy ads, and that its technologies are interoperable with 700 rival ad platforms, and 80 rival publishing platforms.