A U.S. trade body found the Silicon Valley giant had infringed on a patent owned by the Korean company and slapped a ban on the sale of certain older iPhone and iPad models sold by AT&T.
The U.S. International Trade Commission, an independent federal agency, issued a limited order stopping all imports and sales for AT&T models of the iPhone 4, iPhone 3GS, iPad 3G and iPad 2 3G. The versions targeted are more than a year old but are still solid sellers.
All such exclusion orders are sent to President Barack Obama, who has 60 days to review them. If he does not veto the order, it goes into effect.
"We are disappointed that the commission has overturned an earlier ruling and we plan to appeal. Today's decision has no impact on the availability of Apple products in the United States," Apple spokeswoman Kristin Huguet said in a statement.
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Designed to be a trade panel, the ITC has become a popular venue for patent lawsuits because it acts relatively quickly and it can order import bans, which are more difficult to get from district courts.
Samsung said in a statement that the ITC decision "confirmed Apple's history of free-riding on Samsung's technological innovations."
"Our decades of research and development in mobile technologies will continue and we will continue to offer innovative products to consumers in the United States," it said.
Tuesday's ruling overturned a decision by ITC Judge James Gildea, who ruled in September that Apple did not violate patents at issue in the case, which was filed in mid-2011.
Apple was found to infringe on a patent that relates to 3G wireless technology and the ability to transmit multiple services simultaneously and correctly. It is essential to ensuring that the devices are interoperable.
The U.S. Justice Department, the Federal Trade Commission and the Patent and Trademark Office have all said that infringement of these "standard essential patents" should mostly be punished by monetary charges, not sales bans.
An exception would be in the rare instances where the infringer refuses to negotiate a license or to pay.
The usual expectation among companies has been that standard essential patents will be inexpensively licensed to anyone.
Samsung, which is battling Apple in court in some 10 countries, had also accused Apple of infringing on three other patents, but the ITC found that Apple did not infringe these.
Apple has a parallel complaint filed against Samsung at the ITC, accusing Samsung, an Apple chip provider, of blatantly copying its iPhones and iPads. An ITC judge in that case found that Samsung had violated one patent but not a second one. A final decision is due in August.
Apple has waged an international patent war since 2010 as it seeks to limit the growth of Google's Android system. The fight has embroiled Samsung, HTC and others that use Android.
The ITC's decision came on the same day that President Barack Obama weighed in on curbing a totally different type of patent lawsuit - those brought by companies called "patent trolls." The disparaging name is because these companies make or sell nothing, but they specialize in suing others for infringement. Obama asked for new federal regulations on these concerns and action from Congress.
The offensive - announced before Obama makes a fundraising trip this week to California's Silicon Valley - came as U.S. lawmakers and courts are seeking ways to reduce the number of unwarranted patent lawsuits.
The case at the International Trade Commission is No. 337-794.