Microsoft and other firms have complained to the EU about Google+, sources have said.
The EU may broaden its ongoing investigation into Google after the complaints against its social networking tool.
Microsoft's complaint underlines the mounting rivalry between the two companies.
Microsoft and the other companies expressed their concerns informally, the two sources said, declining to provide details about the nature of the complaint about Google+ because of the sensitivity of the matter.
The European Commission declined to comment and Microsoft said it would not make any statement about "rumours or speculation."
Google, the world's most popular Internet search engine, launched Google+ in June last year, pitting it against Facebook, the world's biggest social network, and Twitter, among others.
The U.S. Federal Trade Commission expanded its investigation last month into the company's search activities to include Google+, a source familiar with the matter has told Reuters.
Microsoft last week asked the European Commission to step into a patent dispute with Google and Motorola Mobility, accusing the U.S. mobile phone maker of setting excessive charges for the use of its patents in Microsoft products.
EU and U.S. regulators have given approval for Google to buy Motorola Mobility.
Motorola is also the subject of complaints by Apple, which has asked the EU regulator to take action against the mobile phone firm over patent licensing.