Hundreds of thousands of computers will simultaneously lose access to the internet on July 9, unless their owners check for a virus that may have remained hidden for several years, the FBI has warned.
The so-called "internet doomsday" will come as the bureau turns off a server system its investigators installed in November, when they broke up an Estonian crime ring that was running a global internet advertising scam.
Since 2007, the six men had been using a "Trojan" virus - named after the mythical horse - to seize control of computers all over the world.
The virus, called "DNS Changer", would redirect the web browsers of victims from sites they wanted to visit to ones controlled by the gang.
Revenue from adverts tied to those sites is estimated to have earned the criminals US$14 million ($17.2 million) in commission.
By late last year, when the FBI and Estonian authorities shut down the racket, the virus had seized control of about half a million computers.
However, had the investigators simply closed down the gang's servers, every computer infected would have been unable to access the internet.
The FBI replaced the gang's servers with a surrogate server system, and set up a free website which allows people to check if they are victims of the virus, and then remove it from their machine.
CHECK YOUR COMPUTER