The malware causing the damage is called Reveton, a drive-by attack incorporating the Citadel bank Trojan platform that has caused a since the beginning of 2012. It weaves its spell by threatening victims with prosecution by the US Department of Justice (DOJ) after - it claims - detecting child porn or other illegal activity.

Victims can avoid trouble and regain control of the infected PC by paying a bogus 'fine', an option a growing number of people appear to be choosing.

"While browsing the Internet a window popped up with no way to close it. The window was labeled FBI and said I was in violation of one of the following: illegal use of downloaded media, under-age porn viewing, or computer-use negligence," one victim told the FBI.

"It listed fines and penalties for each and directed me to pay $200 [£128] via a MoneyPak order. The page said if the demands were not met, criminal charges would be filed and my computer would remain locked on that screen."

The FBI's advice is not to pay out under any circumstances and to turn to a security programme or computer support shop to remove the malware.