The investigation centres on whether the firm breaches competition rules by skewing search results in its favour, thereby restricting rival online advertising platforms.
In a statement on Thursday, the European Commission said its preliminary conclusion was that “Google has abused its dominant position by systematically favouring its comparison shopping service in its search result pages”.
It is not the first time that the US company has run into trouble in Europe.
In May, Google’s Paris office was raided by French police as part of an investigation into tax fraud and money laundering.
The company is facing mounting pressure over its tax affairs amid a backlash against corporate tax avoidance by multi-national companies.
Google agreed a controversial £130 million deal with HM Revenue & Customs in January to settle a 10-year tax inquiry into its UK business.