More computer users are falling victim to fake anti-virus software that tricks them into paying phony subscription fees and divulging personal information, according to a study by Google.

Over 13 months, Google analyzed 240 million web pages to study the prevalence of the problem and how cyber criminials target and distribute malicious software or malware.

It found that fake anti-virus software now accounts for 15 per cent of all malicious software on the web, up from three per cent when the study began in January 2009.

Typically, cyber criminals try to convince users that their computer systems are infected and offer a free download to scan for malware, the study said. They pretend to scan computers and claim to find infected files — files which may not even exist or be compatible with the computer’s operating system.

Users are forced to register the fake program for a fee in order to make the warnings disappear.

"Surprisingly, many users fall victim to these attacks and pay to register the fake AV," the study said. "To add insult to injury, fake AVs often are bundled with other malware, which remains on a victim's computer regardless of whether a payment is made."

Fake attacks occur frequently through popular websites that reach more users, including spam websites and online ads, the study said. Facebook, the New York Times and Twitter have all been used to distribute fake software.

More than 11,000 web domains were involved in the distribution of fake AVs, the study concluded.

Currently, they are responsible for 50 per cent of all malware delivered by ads for the fake software, which represented a five-fold increase from a year ago, the study said.

"Despite continuously improving detection and mitigation techniques, fake AV attacks continue to persist, demanding increased awareness and broader response from the research community at large," the study concluded.