Gangsters are tricking worried internet users into buying anti-virus protection that is actually malicious software in disguise, security experts warned today.
Investigators fear hacking gangs are pocketing millions of pounds by infiltrating customers' computers and stealing sensitive banking details.
They said criminals pose as legitimate IT companies who cold call victims offering fake security software that can be downloaded for around £30.
The crooks then combine credit card information from the sale with stolen personal information to defraud the customer or commit further crimes.
Sharon Lemon, who is responsible for fighting cyber crime at the Serious and Organised Crime Agency (Soca), said it is "big business".
She said: "In recent cases, we have seen gangs employing 300 to 400 people to run their operations and using call centre-scale set ups to target victims en masse.
"They can also be paying out as much as 150,000 US dollars a month (on a pay per download basis) to individual webmasters who are unwittingly advertising their fake software - this level of investment from criminals indicates that the returns are much heftier than this."
The warning came at the start of an internet security awareness week organised by GetSafeOnline.org, which is supported by Government bodies, police and private companies.
Research conducted on behalf of the campaign group found one in four adult web users in Britain have been approached by someone offering to check their computer for viruses.
Thousands of spam emails offering virus check services have also been sent out while almost half (48%) of all web users have seen a pop-up window claiming their computer is infected.
Last month, a computer security expert Matthew Anderson, 33, of Drummuir, Aberdeenshire, admitted being a key member of an international hacking group engaged in a similar scam.
He was caught after an investigation by Scotland Yard and authorities in Finland into a gang, known online as the m00p group, who wrote computer viruses to order that were then attached to spam emails.
Government minister Baroness Neville-Jones said: "While it's encouraging to see that UK web users are today more security-aware, criminals will always try to be ahead of the game and will use increasingly sophisticated methods to take advantage where they can.
"However, equipped with the right information, there's no need for anyone to be deterred from going online or from protecting their computers with the right security software."
Tony Neate, of getsafeonline.org, said: "Web users should ignore cold calls from companies offering free virus checks, and be very cautious of any on-screen pop ups.
"Most reputable IT providers do not approach customers in this way without prior notice or a direct request."
:: More information can be found on http://www.getsafeonline.org.