The US International Trade Commission (ITC) issued a preliminary ruling that Microsoft infringed on Motorola Mobility patents in its hit Xbox 360 videogame consoles.
Administrative law judge David Shaw on Monday backed four out of five Motorola claims that the company's intellectual property was usurped in Xbox software for tasks such as efficiently moving video files or connecting wirelessly to the Internet.
"Today's recommendation by the administrative law judge is the first step in the process leading to the Commission's final ruling," Microsoft said in an email response to an AFP inquiry.
"We remain confident the Commission will ultimately rule in Microsoft's favor in this case and that Motorola will be held to its promise to make its standard essential patents available on fair and reasonable terms."
A final ruling from the quasi-judicial federal agency is expected later this year.
An ITC ruling against Microsoft would force the Redmond, Washington-based company to work out a licensing deal with Motorola or risk having its top-selling videogame console banned from US stores.
Explore further: Nokia posts higher Q3 profits on one-off tax gain