The EU has formally charged Google with abusing its search market position in Europe, leaving it open to a fine of more than $6bn (£4bn).
The European Commission has been examining whether Google, which holds about 90% of the search market in Europe, has been illegally rigging its search results to favour its own services.
Tech rivals such as Microsoft, who urged the EU to bring the case, want more competition in areas like online maps, search and shopping.
EU competition commissioner Margrethe Vestager said Google has given "an unfair advantage to its own comparison shopping service".
Rivals object to the firm placing adverts for its Google Shopping service ahead of other links in relevant searches.
The EU has issued a statement of objections which Google has 10 weeks to respond to before action can be taken.
Ms Vestager said that a separate antitrust investigation has been ordered into Google's mobile operating system Android.
She said: "In the case of Google I am concerned that the company has given an unfair advantage to its own comparison shopping service, in breach of EU antitrust rules.
"Google now has the opportunity to convince the Commission to the contrary. However, if the investigation confirmed our concerns, Google would have to face the legal consequences and change the way it does business in Europe."
An internal Google memo informed staff that the company believes it has a "strong case". In a blog post the tech giant used a series of graphs to show that competition continues to thrive.
The company has repeatedly denied any wrongdoing. It could face an eventual fine of up to 10% of its worldwide turnover, which reached $66bn (£44.7bn) in 2014.
The filing of charges may increase pressure on Google to settle, to avoid a potentially damaging case and massive fine resulting from the allegations.