Microsoft and several other companies have complained to EU antitrust regulators about Google's social networking tool, two people familiar with the matter said, but they declined to provide the names of the other firms involved.

The complaints, which have not yet been filed formally with the Commission, may prompt the European Commission to broaden its ongoing investigation into Google, which focuses on whether it is too dominant in the web-search market.

Google declined to comment, as did the European Commission. Microsoft's spokesman Jesse Verstraete said Microsoft had not filed a complaint with the Commission.

The sources declined to provide details about the nature of the complaints about Google+ or the names of the other complainants because of the sensitivity of the matter, and emphasized that the process was informal at this stage.

Microsoft's latest grievance underlines the intensity of the rivalry between the two technology giants - it has already made formal complaints to EU regulators about Google on two other issues in the past year.

Google, the world's most popular Internet search engine, launched Google+ last June, pitting it against Facebook, the world's biggest social network, and Twitter, among others.

Facebook had 55.5 percent of global Internet users last month while Microsoft's Windows Life Profile and Google+'s share came to 6.3 percent and 6.1 percent respectively, according to online data tracking service comScore.

Google+ allows users to play games and share photos with friends, incorporates a video chat facility and offers a more personalized search service by integrating it with the original Google engine.

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.

EU Competition Commissioner Joaquin Almunia reiterated on Tuesday he would decide by the end of March or early April the next step in the regulator's ongoing investigation into Google, and charges by rivals that it misused its market position.

We continue to receive information and complaints or informal complaints, so it's not a simple task to investigate, Almunia told lawmakers at the European Parliament.

The Commission launched the investigation in November 2010 after rivals, including Microsoft, accused Google of abusing its dominant position in the market for web search services. There are now 12 complaints.

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.

(Reporting by Foo Yun Chee; editing by Rex Merrifield, Hans-Juergen Peters and David Hulmes)

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