The report, quoting a study by Click Forensics, a San Antonio-based consulting service, said sales referrals generated by clicks on the brief advertising links popularized by the two Internet powerhouses were a sham 14.1% of the time, based on information collected from 1,300 online marketers.
This was up from a click fraud rate of 13.7% three months ago, according to the study.
The statistics agree with other data asserting advertisers were paying a significant sum to Google, Yahoo and their partner Web sites for phantom shoppers even as more resources are devoted to thwarting scammers, the report said.
A recently released survey of 407 online advertisers by market research firm Outsell estimated that click fraud cost advertisers $800 million last year.
The report said click fraud was a highly sensitive subject for Google and Yahoo because it raised doubts about the trustworthiness of the advertising model that drove their profits and stock prices.