SAN ANTONIO, TEXAS - A Texan click-fraud monitoring service says around 14% of clickthroughs to ads sold by internet search engines are bogus.

Click Forensics' figures suggest leading search engines such as Google and Yahoo have lower click-fraud rates - 12.8%, compared to 20.3% and 27.1% for second and third-tier search providers.

Search engines favour a "pay-per-click" business model, where advertisers bid to pay so much per click for their ads to figure prominently when users search for terms related to their businesses.

Click fraud can occur when a competitor clicks on ads to drain a rival's marketing budget or an affiliate site clicks on the ad to earn a percentage of what the advertiser pays the search engine.