Up to 300,000 computer users around the world are at risk of losing internet access due to a computer virus.
Cyber criminals 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 tomorrow and anyone who has not removed the offending software by then will not be able to access the web.
Some blogs and news reports hyped the risk of an outage, warning of a potential "blackout" and describing the Alureon malware as the "internet Doomsday" virus.
Yet experts said only a tiny fraction of computer users were at risk, and internet providers would be on call to quickly restore service.
They said they considered the threat to be small compared with more-prevalent viruses such as Zeus and SpyEye, which infect millions of PCs and are used to commit financial fraud.
The viruses were designed to redirect internet traffic through rogue DNS servers controlled by criminals, according to the FBI.
When authorities took down the rogue servers, a federal judge in New York ordered that temporary servers be kept in place while the victims' machines were repaired.
The temporary servers shut down today, meaning the infected PCs that had not been fixed would no longer be able to connect to the Internet.
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."