The Business Software Alliance (BSA) claims to have prevented over 36,000 illegal software products being sold by online auction sites during the first six months of 2007.

The retail value of the software identified during this period came to over $8m, according to the anti-piracy group's calculations.

John Wolfe, director of internet enforcement at the BSA, reckons that this amount is just the "tip of the iceberg".

"This represents auctions we have identified as infringing our members' copyrights and taken action to have them closed by the auction site owners," he said. "Some auction offers may lead to dozens or even hundreds of illegal software purchases."

Microsoft won £35,000 in damages against a counterfeit software seller this week who admitted selling counterfeit software on channels such as eBay and to illegally importing software.

The BSA highlighted a recent study by IDC which revealed that the chances of buying software on an auction site that has not had malware embedded into the code is less than one in two.

"People buying software on auction sites need to be concerned if prices appear too good to be true," said Wolfe.

"Counterfeit copies may not give you the functionality and full benefits of a legal version, and there is a significant data protection risk in that counterfeit software may be linked to hackers looking to access your network."

As well as widely available commercial applications, the BSA also found high-end commercial software used only by businesses, such as products developed for computer-aided design, being offered illegally online.

"Businesses purchasing their software via these online offers should use caution to avoid being duped or unwittingly introducing viruses or spyware onto their networks," warned Wolfe.

"They also face the legal and financial risks associated with violating intellectual property laws by installing unlicensed software."

The BSA also said that it finds more than 200,000 illegal software files available on file-sharing networks in an average month.