Internet service provider Virgin Media has warned more than 1,500 of its customers that their PCs have been infected with a malicious virus that steals online banking logons.
Letters have been sent by the ISP to broadband customers who have fallen victim to the SpyEye Trojan, advising them on how to remove the malware from their systems.
It's understood to be the first time a UK ISP has issued customers with a specific warning over malware, rather than offering generalised advice or providing antivirus software.
Virgin claims not to have been monitoring users' activity, but said it had acted on a tip-off from the UK police's Serious and Organised Crime Agency, which discovered the infections while researching criminal botnets and passed details of the victims' IP addresses to the ISP.
In addition to offering advice on removing the malware, the letters provide information on a help service to which customers can sign up that will allow Virgin to remotely find and fix problems.
In total, the ISP has issued 'several thousand' alerts to individual users since launching the service last August.
"It's a small number compared to the four million customers we have," a Virgin spokesman told the BBC, "but regardless of that, because of the seriousness it's still important to communicate with our customers."
SpyEye first appeared in early 2010, and has spread via infected websites, or by tricking users into clicking on links to install the malware.
The Trojan was created using a software development kit, available online for $500 (£310), which enables virus creators to control a botnet of all infected PCs.