Up to 300,000 people are at risk of finding they cannot use the internet from Monday after cyber criminals infected thousands of computers.
The issue, which has affected computers around the world, came about after the gang created a piece of malware named DNSChanger, which infected computers and redirected them to malicious websites, making those behind it millions of pounds through ad hijacking.
Since then the FBI has continued to run the servers for a grace period to give computer users time to remove the infection from their machines.
This period ends on Monday and anyone who has not removed the offending software by then will not be able to access the web.
Marcin Kleczynski, chief executive of Malwarebytes, which makes a free piece of software designed to remove this kind of malware, said: "The FBI left the criminal servers running to give infected users time to remove this piece of malware, however it is estimated there are still hundreds of thousands of people who just don't know they are infected.
"Monday is D-day because anyone who still has this piece of malware on their computer simply won't be able to access the internet.
"This means people will struggle to repair the problem because they won't be able to download removal tools or access information through the infected laptop or computer.
"The total cost could be significant."
A FBI spokesman said: "We've been using the last eight months to go out and clean up the infected computers, but we don't have everybody."