A virus which spread almost a year ago could cripple 20,000 UK computers on Monday, experts have warned.
The purpose of the virus was to change victims' internet settings to make them visit fraudulent websites.
The DNSChanger botnet was installed on hundreds of thousands of machines until the FBI in the US managed to uncover the source of the fraud about 12 months ago.
However because the virus needs to access the botnet's servers to keep infected users online, the FBI were unable to shut it down right away. If they had, thousands would have lost internet access immediately.
Instead they have had to leave the virus' servers online for more than a year - and a planned switch-off in March also had to be delayed because hundreds of businesses still hadn't cleaned their machines.
Around 20,000 people in the UK and hundreds of thousands in the US are still thought to be infected.
But on 9 July, the FBI will be turning the botnet off for good - and that could leave thousands stranded.
Users have been warned by several sources, including Google, Facebook and ISPs, that they need to run anti-virus software on their machines and remove the virus before it is too late.
But many have ignored the warnings and are now facing 'Internet doomsday'.
Internet security firm Kaspersky has even released a free tool to uninstall the virus, and has encouraged anyone still affected to do so before Monday.