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 others 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.

(Reporting by Foo Yun Chee; editing by Rex Merrifield)