In a statement, Competition Commissioner Margrethe Vestager said the U.S. tech giant, which dominates Internet search engines globally, had been sent a Statement of Objections – effectively a charge sheet -- to which it can respond.
"I am concerned that the company has given an unfair advantage to its own comparison shopping service, in breach of EU antitrust rules," she said. "If the investigation confirmed our concerns, Google would have to face the legal consequences and change the way it does business in Europe."
The Commission, whose control of antitrust matters across the wealthy 28-nation bloc gives it a major say in the fate of global corporations, can fine firms up to 10 percent of their annual sales -- or a penalty of over $6 billion for Google.
If it finds that companies are abusing a dominant market position, the EU regulator can also demand sweeping changes to their business practices, as it did with U.S. software giant Microsoft in 2004 and chip-maker Intel in 2009.
There was no immediate public response from Google, but an internal memo to staff published by the blog re/code described the moves as "very disappointing news" and said: "We have a very strong case, with especially good arguments when it comes to better services for users and increased competition."

Of the formal investigation into Android, used on smart phones and tablets, Vestager said: "I want to make sure the markets in this area can flourish without anticompetitive constraints imposed by any company."
She announced the moves on the eve of a high-profile visit to the United States, following five years of investigation and abortive efforts to strike a deal with Google by her Spanish predecessor, Joaquin Almunia, who handed over the politically charged dossier to the Danish liberal in November.
However, the focus on the ranking of searches for shopping sites -- Google has its own service called Google Shopping – did not address all the complaints lodged with the Commission by competitors, large and small, in Europe and the United States, who say Google has hurt their business.
Google initially has 10 weeks to respond to the charges and can demand a hearing. A final resolution -- quite possibly involving court action if Google does not choose to settle – is likely to take many months and probably years.