SAN FRANCISCO (AFP) - Click fraud surged to a new high in the final three months of 2008 as criminals had armies of hacked computers pretend to be customers checking out online ads, according to a report released last week.

Click Forensics, a US firm that audits Internet traffic, reported that 17.1 percent of clicks on online advertising were frauds evidently intended solely to drive up bills for businesses paying “per click.”

The rate was a half of a percent higher than the one reported in the same quarter a year earlier.

Networks of hacked computers referred to as “botnets” are said to be responsible for nearly a third of the click fraud in final three months of 2008.

“It seems that the online advertising industry is not immune to the growing tide of cybercrime during this recessionary period,” said Click Forensics president Tom Cuthbert.

“Both the overall click fraud rate and the rate of click fraud originating from botnets were the highest ever in Q4 2008. Advertisers should pay close attention to these types of threats.”

The 28.2 percent rate of fraudulent clicks on advertising at Internet search engines, including Google AdSense and Yahoo Publisher Network, represented a tenth of a percent drop from that reported in the final quarter of 2007.

Internet firms including Yahoo and search king Google rely heavily on revenue from pay-per-click ads and employ technology to expose bogus clicks so advertisers don’t have to pay.

Click Forensics said motives for fraud include pumping up commissions earned on ads and driving up expenses for competing businesses.