Motorola Mobility – the company that Google is buying for $12.5bn (£8bn) – has infringed a Microsoft patent in making its Android handsets, according to a ruling by the US International Trade Commission.

The decision was tweeted by Brad Smith, Microsoft's general counsel, who leads its legal team.

The verdict – which is an "initial determination", rather than a final verdict – follows a complaint filed in October 2010 by Microsoft in which Motorola was accused of infringing nine patents relating to the now-discontinued Windows Mobile and the current Windows Phone platforms.

Microsoft subsequently dropped two of the patents from the list. The patents complained of cover topics such as monitoring remaining memory, updating contact lists and synchronising on- and offline use.

Motorola was found to have infringed on four claims relating to US patent 6,370,566 which covers "generating meeting requests and group scheduling from a mobile device", filed in 1998 and granted in 2002.

"ITC finds Motorola patent infringement in Microsoft case. Another indication that licensing is the best path for the industry," tweeted Smith. The ITC has published its finding.

Motorola was cleared of infringement of the six five other patents. In a press release, Scott Offer, Motorola's senior vice president and general counsel, said: "We are very pleased that the majority of the rulings were favourable to Motorola Mobility. The initial determination may provide clarity on the definition of the Microsoft 566 patent for which a violation was found and will help us avoid infringement of this patent in the US market."

The trade commission is a quasi-judicial arbiter that can block imports of products found to infringe US patents. The agency has become a venue of choice for patent grievances since it typically completes investigations faster that US courts.

A judge is due to rule on a similar patent infringement case brought by Apple against Motorola on 13 January.

The outcome presents a significant problem for both Google and Motorola. The US is currently the largest smartphone market in the world, and the ITC ruling could allow Microsoft to apply for a ban on sales of Motorola smartphones there unless it pays a licence fee to Microsoft.

But for Google, which has been understood to be buying Motorola in order to bolster its patent portfolio, the vulnerability of its acquisition to Microsoft's patent portfolio may mean that it cannot protect other Android licencees from the need to pay licence fees to Microsoft to cover their liabilities under patent law.

Microsoft is already receiving millions of dollars in payments from Samsung and HTC after settling patent litigation related to Android.

Google made its bid for Motorola Mobility, which was spun off as an independent unit in January this year, after failing to acquire a raft of patents from the bankruptcy of Nortel in June.

It began negotiations to purchase Motorola Mobility within days, following a meeting between Larry Page and Andy Rubin, of Google, and Sanjay Jha, chief executive of Motorola Mobility.

In an interview on 12 December, Google chairman Eric Schmidt said the ITC is "being used by some companies to try to shut down choice". He said: "It's bad… From a consumer perspective, you want choice and the consequence of this ITC mechanism is that it appears, if it does find against Android, it could limit your choice."

Florian Müller, an independent patent analyst, said: "The ones who can actually sit back and relax as they watch this are those who have concluded license deals with Microsoft (or other patent holders) and don't have to worry about possible or actual import bans, possible or impossible workarounds, or further escalation."

Motorola meanwhile has a case in train against Microsoft's Xbox.

Apple earlier on Tuesday won a similar determination against HTC over its Android handsets, and could see a ban implemented by April unless HTC can find a way to work around its infringement.

The HTC ruling yesterday, which didn't go as far as Apple sought, should give the iPhone maker "a stronger position" in its intellectual-property cases against other Android-device makers, Brian Marshall, an analyst at ISI Group in San Francisco, said in a note to clients.

"With about $80bn in net cash, we believe Apple is not interested in a financial settlement with HTC or other Android vendors but wants to stop the shipment/sale of products that infringe on their vast IP portfolio," Marshall said.

Motorola, meanwhile, last week won a similar injunction against Apple in Germany over wireless patents which it could use there to force the iPhone and iPad off shelves – though Apple has said it will appeal.

Update: this article was corrected after publication.

• Motorola was found to have infringed only one patent, rather than four: it infringed on four of the claims covered by Microsoft's 566 patent, rather than four separate patents.

• Motorola was cleared of infringement relating to six Microsoft patents, not five.