The International Trade Commission (ITC) has ruled that Apple infringed one of Samsung's patents, which could mean some older models of the iPad and iPhone will be banned from sale in the US.
The ruling overturns an earlier ITC ruling that said Apple did not violate the patent at issue in the case, which was filed nearly three years ago.
The patent relates to 3G wireless technology and the ability to transmit multiple services at the same time.
The ITC ordered a halt on all imports and sales on AT&T models of the iPhone 4, iPhone 3, iPhone 3GS as well as the iPad 3G and iPad 2 3G, although some of these devices are no longer sold in the US.
Apple said it plans to appeal and the ruling could also be reversed by a US presidential order in 60 days, although the use of this power is rare, according to the BBC.
Samsung welcomed the new ruling.
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"We believe the ITC's final determination has confirmed Apple's history of free-riding on Samsung's technological innovations," Samsung said in a statement.
Apple is free to continue selling the models involved during the 60-day review period, and the company was quick to say the ruling would have no impact on the availability of Apple products in the US.
The ruling is the latest development in the longstanding battle between Apple and Samsung, which are fighting patent dispute cases in ten countries.