The software company Microsoft Corp is suing Motorola Inc., one of the biggest backers of Google‘s Android software, claiming the cellphone maker is infringing nine patents in handsets powered by Android.

While Microsoft has struggled to win adoption of its Windows software for phones, Android, which Google gives away free, has been widely adopted by handset makers and software programmers.

The suit comes as Microsoft is set to introduce its latest operating system for smartphones that can compete with Apple Inc.’s iPhone and devices running Android.

Motorola, which has stopped making phones running Microsoft’s software, has bet its future on Android. The company is preparing to split itself apart, establishing an independent cellphone business.

Motorola said it hasn’t received a copy of the complaint, which was filed in federal court in Seattle, but added: “The company will vigorously defend itself in this matter.”

Google’s mobile software is at the heart of other legal challenges. Apple has sued an Android phone maker, HTC Corp. for alleged patent infringement, while Oracle Corp. has sued Google directly.

If they’re successful, the lawsuits could help raise the costs for companies that use Android.

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