Yahoo has promoted one of its attorneys to the newly-created position of "Click Fraud Tsar ". 

Reggie Davis becomes the search giant's first vice president of marketplace quality, after seven years on its team of legal staff.

Click fraud can occur when a search engine's ad system is tricked into counting ads never seen by real users, or when automated programs generate fake clicks.

Davis has been handed responsibility for reducing the problem, and for increasing trust among advertisers with Yahoo, the world's second-largest search provider.

Davis revealed that Yahoo's discard rate of invalid clicks averaged 12-15 per cent. Google claims that its discard rate is less than 10 per cent.

Click fraud analyst firm Click Forensics welcomed Davis' promotion. "Yahoo is taking a more honest approach than the others in talking about the scale of the problem," said Click Forensics president Tom Cuthbert. 

"But there is still a big disparity between [invalid] clicks and what advertisers are paying."

Google plans to introduce a new AdWords service next month that aims to reduce click fraud by allowing clients to block certain users from seeing their ads.