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"We've received a report that copyrighted music has been shared using a computer linked to your Virgin Media Internet account," say the letters.
Virgin Media told customers they needed to ensure copyrighted files are not downloaded or shared from Virgin Media internet connection in future and that they must secure any wireless router they have.
The letter recommended that subscribers install Virgin's own security software to make sure "no further steps" were taken against them.
The British Phonographic Industry (BPI), the record industry's trade association, is working with ISP Virgin Media to identify computers they believe are sharing music illegally. The BPI is identifying computers by monitoring peer-to-peer networks.
"The BPI simply connects to a host computer offering files and downloads them in the same way that any other peer-to-peer user would, capturing evidence including the user's IP address, as it undertakes the download," said the British Phonographic Industry.
The evidence collected by the BPI is made available by any uploader in the normal course of using a p2p network, said the BPI.
"Without going into precise technical details on how the information is collected and documented, the BPI logs on to the network as a peer and initiates a download from the IP address, using a semi-automated process," it said.
The BPI sends the evidence to their ISP, who can identify that customer from the IP address and send them an advisory letter.