Microsoft has filed an anti-trust complaint against Google's European operations.
The software maker says Google has abused its dominant position in internet search by giving less prominence to commercial websites that offer similar services to its own.
It cites a number of practices, including Google limiting the ability of Microsoft Bing to index web content.
In a statement, Google said it would cooperate with any investigation.
The European Commission began an investigation into accusations of anti-competitive practices by Google last November, at the behest of several internet companies, including Ciao, a shopping site owned by Microsoft.
If the commission accepts the latest round of complaints from Microsoft, the BBC reports it is likely they will be rolled into the same investigation.
Penalties for companies found to have engaged in anti-competitive practices in Europe can be severe. The EC has the power to impose fines of up to 10% of global earnings.
