Last month Microsoft took its case to the European Commission, saying Motorola Mobility was charging too much for use of its patents in Microsoft products.

Motorola Mobility said Apple had also complained to the EU watchdog about its patents.

Google Inc (GOOG.O), which is in the process of buying Motorola Mobility, has said it will offer Motorola patents on fair and reasonable terms once the deal is completed.

The EU watchdog, which is also investigating whether legal tactics used by Samsung Electronics Co Ltd (005930.KS) against Apple breach EU antitrust rules, may open a second patent case into the sector, EU Competition Commissioner Joaquin Almunia said.

"I am considering whether we need to investigate these complaints formally to help bring more clarity into this area of competition control," Almunia said in the text of a speech to be delivered at a conference in Washington.

"The holders of standard-essential patents have considerable market power. This market power can be used to harm competition ... I don't need to tell you that this is unacceptable, and I am determined to use antitrust enforcement to prevent such hold-up by patent holders," he said.

The Commission, which can fine companies up to 10 percent of their global turnover, has levied billions of euros in penalties against Microsoft, Intel (INTC.O) and others for breaching EU rules.

(Reporting by Foo Yun Chee; Editing by David Holmes)

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