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.
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.
In 2011, an international group of law enforcement agencies, including the FBI, arrested the group operating DNS Changer malware botnets. But hundreds of thousands of computers remain infected and are currently using interim systems set up by the FBI to access the internet.
But after 9 July, 2012, all computers still infected with DNS Changer malware will no longer be able to access websites, e-mail, chat or social networking sites such as Facebook, when the temporary systems will be disabled because of high operating costs.
The DNS Changer Working Group estimates that there are more than 350,000 devices still infected with DNS Changer, out of the 4 million originally affected by the malware.
When the FBI turns off the temporary access systems, it could leave infected machines without access to the web.
It is estimated 20,000 of these machines are in the UK, according to the Telegraph.
In June, Facebook joined the DNS Changer Working Group (DCWG), aimed at cleaning up the malware.
As a result of Facebook's work with the DCWG, the social networking site said it is now able to notify users likely to be infected with DNS Changer malware and direct them to instructions on how to clean their computer or networks.
Facebook's decision to notify users who have infected computers follows a similar decision by Google, which started alerting users of DNS Changer infections in May.