Virgin Media has sent around 800 letters to customers warning them to stop sharing music on file-sharing sites.

The letters are part of a 10-week campaign the internet service provider (ISP) is running in conjunction with the recording industry body, the BPI.

Virgin said the letters, which list the track that has been both uploaded and downloaded, along with the date and time, would help educate users on the possible penalties people face.

However, the envelope for the letters has a threat to disconnect repeat offenders printed on them.

Virgin denied it was the start of the company implementing a three-strikes system. This is where users of file-sharing networks get two warning letters to stop them downloading or uploading music or movies. If users persist their broadband service is disconnected. It has been adopted in France and is something that the BPI is pushing for UK ISPs to sign up to.

A spokesman for Virgin Media said: “The letters should have been sent out in a plain envelope and the printed warnings were an administration error.

"This is about education. We make no assumptions about who is at fault. It may be someone in the family or someone illegally using their Wifi connection, " he said.

Virgin Media is given people’s information by the BPI, which monitors file-sharing networks. It initiates a track to download from the IP address of the file sharer it is watching. It then sends the information to Virgin, which can identify the customer concerned from the IP address.

The BPI said there was no “spying” involved in the collection of data. It said in a statement: “The evidence collected by the BPI is in fact made available by any uploader to in the normal course of using a P2P (filesharing) network. Nor does the process raise “data protection issues".

However, privacy groups are concerned and point out that the scheme could have unintended consequences for Virgin.

Dr Yaman Akdeniz, director for the Cyber Rights Group, said: “Virgin is sending out these threatening letters which will upset lot of people.

“These threatening 'Big Brother is watching you' tactics will only push users to other ISPs rather than acting as a useful educational tool.”