20,000 PCs in the UK could be cut off from the web on Monday 9 July 2012, as a consequence of the DNS Changer virus.
Infected machines will no longer be able to access websites, e-mail, chat or social networking sites such as Facebook, according to the FBI.
Worldwide, 350,000 computers could lose web access on Monday because of the DNS Changer virus, the FBI has warned.
The malware, a variant of the Zlob family of Trojans, is designed to tell the infected computer to use a rogue Domain Name System (DNS) server, which directs the browser to hacker-owned websites.
Infected machines then replace legitimate advertisements with ads the criminals try to monetise via click fraud. The cyber criminals are thought to have made ¬£9.1m from the scam.