Updated BT, the UK's largest broadband provider, has begun threatening subscribers with disconnection from the internet if it is told they are sharing copyright music over peer-to-peer networks, The Register has learned.

The firm recently sent an email to one of its four million retail broadband customers, who asked not to be named, alleging that she had illegally participated in a network sharing of Biology , a song by Girls Aloud.

The email reproduces evidence collected by the BPI. It purports to show she used the open source filesharing program Ares in May this year to infringe sound recording copyright. Ares can be used as a client for both Gnutella and BitTorrent networks.

Geoff Taylor, chief of UK record industry trade body the BPI, told The Register in a statement today: "Establishing partnerships with ISPs is the number one issue for the BPI, and we are beginning to form positive working relationships with BT, Virgin Media and most of the other major ISPs."

It's unclear whether BT has agreed to formally implement the record industry's preferred "three strikes" procedure that would see those accused of infringing music copyright warned twice and suspended or disconnected from the internet.

Taylor continued: "Everyone agrees on where we need to be, and we are working closely with our colleagues across the music community, the more progressive ISPs, and government to get us there."

BT said: "We don't comment on commercial relationships and communications with individual customers." A spokesman said BT broadband customers who are infringing copyright over peer-to-peer networks can expect a similar threat if the BPI provides evidence against them.

Accusations and evidence

The BPI evidence BT shared with its customer consists of the Ares user agent, a timestamp, a file name and an IP number. BT's letter, from a member of its "Customer Security Team" states: "I have received a complaint regarding one of our customers offering copyrighted material over the internet. On investigation, I have found that your account was used to make this offer."

Collecting this kind of evidence does not require ISPs to monitor their customers' internet connection. BPI investigators are simply able to collect lists of IP numbers participating in copyright-infringing peer-to-peer networks and trace which operator they belong to. Assuming the ISP has agreed to do so, it can then identify the individual account holder without sharing personal information with the BPI.

Committed downloaders are able to take technical counter-measures to dodge detection, but the record industry is hoping to win back the mass market - it knows the hardcore are lost for good.

The BT letter goes on to threaten that if the customer continues to fileshare illegally, her broadband account will be shut down: "Sorry, but we're obliged to point out that further similar problems may have to lead to the termination of your account, as such activity contravenes BT's Acceptable Use Policy." It recommends that she ensure her Wi-Fi connection is secure, remove all filesharing software from her computer, and pass the warning on to the rest of her household.

In the BPI letter forwarded to the customer by BT, the trade body says it will look out for further illegal filesharing on her account. "If further evidence is obtained of infringement via your internet connection," it writes, "then further action is likely to be taken against you. That action may include litigation against you, as well as the suspension by BT of your internet connection."