The US law enforcement agency FBI will block up to 500,000 users around the world who have a malicious programme installed on their computer, it was reported here Monday.
It will affect anyone who inadvertantly downloaded a piece of "malware" called DNSChanger, which redirects users to fake advertising websites, the Daily Mail reported.
The Federal Bureau of Investigation (FBI) is taking the unusual step in a bid to shut down DNSChanger, thought to have been created by Eastern European cyber-criminals.
It is not known how many computers in Britain are affected, but 12 percent of the top 500 US companies are said to have infected PCs or Macs, the newspaper said.
The FBI arranged for a private company to run a website - http://www.dcwg.org - as a place where computer users could go to see if their computer was infected and find links to other computer security business sites where they could find fixes for the problem.