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, in Google's case up to 6.2 billion euros.
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 US software giant Microsoft in 2004 and chip-maker Intel in 2009. Its record antitrust fine was 1.09 billion euros on Intel.
Competition lawyer Bertold Baer-Bouyssiere at DLA Piper in Brussels said: "Even more than Microsoft, this case will shape the landscape for the digital sectors in the years to come."
"Not anti-American"
Rejecting suggestions - recently from President Barack Obama himself - that the EU was pursuing an anti-American, protectionist policy, Vestager, a liberal former economy minister, said about a quarter of the firms which had complained to the EU authorities about Google were themselves US-owned.
She insisted that political pressure had played no part in her decision to accuse Google. Nationality or successful market domination were not issues for her, only the abuse of market power.
In a mark of the emotion US tech hegemony evokes among Europeans who fear economic eclipse, the EU parliament last year voted a non-binding resolution urging the Commission to consider breaking up Google. The EU action so far is unlikely to have such a dramatic effect, though it could crimp future business.
Google now had a chance to explain itself, Vestager said, and the case might be settled by it making further commitments to change its products. She wanted a change in the "principle" underlying searches rather than a redesign of current Web pages or tweaks to algorithms by which Google ranks results. That way, any remedy would be "future-proof" against technological change.
Of the investigation into Android, which analysts said could prove a bigger threat to Google's future profitability, Vestager said: "I want to make sure the markets in this area can flourish without anticompetitive constraints imposed by any company."
The focus on the ranking of searches for shopping sites did not address all complaints lodged with the Commission. Vestager stressed her antitrust staff would continue to investigate other areas of concern, including alleged "web scraping" to copy rivals' content, and restrictive practices on advertizing.
A final resolution - quite possibly involving court action if Google does not choose to settle - is likely to take many months and probably years, legal experts said.