A High Court judge has ruled that R4 Magicards, which allow pirated games to be played the Nintendo DS, are illegal in the UK.
Nintendo took legal action against Playables Limited and Wai Dat Chan, which manufacture the £10 devices.
Users can download games from the web onto the cards, which fit into the game cartridge of the Nintendo DS and are not affected by the technology Nintendo builds into the consoles to stop users playing illegally downloaded games.
"The mere fact that the device can be used for a non-infringing purpose is not a defence," said High Court judge Justice Floyd in a answer to claims by card manufacturers that they were designed to ensure homemade games can be played on the consoles.
"The economic effect on Nintendo of the trade in these devices is substantial as each accused device can store and play copies of many Nintendo DS games."
The ruling makes it illegal to import, advertise and sell the cards in the UK
"The court affirmed that game copiers first circumvent Nintendo's security systems before any non-infringing application can be played on Nintendo's handheld products," Nintendo said.