Hundreds of thousands of internet users around the world could lose their connectivity if they fail to check their devices for malware.
(Malware could have taken over their machines more than a year ago and the number of computers that may be infected is more than 277,000 worldwide, although it is down from about 360,000 in April.
It all began when hackers took control of more than 570,000 infected computers around the world. Last year, the FBI set brought in a private company to install two clean internet servers to take over for the malicious servers, so people would not suddenly lose internet access, but that temporary system will be shut down at 4.01am UK time on Monday.
To check whether a computer is infected, users can visit a website run by the group brought in by the FBI: http://www.dcwg.org.)