Eastern European hackers are suspected of placing the Asprox virus on more than a thousand British websites, including those run by the NHS and a local council, in the past two weeks.
Experts described the Trojan.Asprox virus as an alarming departure from commonplace viruses, which tend to be spread through rogue e-mails and unregulated websites.
Unlike other viruses, Asprox sits undetected on mainstream sites, with any visitor at risk of being infected. The virus automatically installs itself on a visitor's computer, allowing a hacker to access financial information.
Last week, Asprox infected the Norfolk NHS website, used by thousands of people a day. Hackney Council's website was one of 12 local council websites also compromised, meaning that anyone logging on to pay a parking ticket or council tax was at risk over a three day period.