Microsoft and several other firms have complained to EU antitrust regulators about Google's social networking tool, two people familiar with the matter said, in a move that may prompt the EU to broaden its ongoing investigation into Google.
Microsoft's complaint underlines the mounting rivalry between the two companies.
Microsoft and the other companies expressed their concerns informally, the people 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 "rumors 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.
Last month, the U.S. Federal Trade Commission expanded its investigation 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.