Swindlers have stepped up their effort to fleece millions of dollars from online advertisers who use lucrative marketing networks run by Google Inc. and Yahoo Inc., according to a quarterly report to be released Monday.

The sales referrals generated by clicks on the brief advertising links popularized by the two Internet powerhouses are a sham 14.1 percent of the time, based on information collected from 1,300 online marketers.

Thats up from a click fraud rate of 13.7 percent three months ago, according to Click Forensics, a San Antonio-based consulting service that compiles the index.

The statistics jibe with other data asserting advertisers are paying a significant sum to Google, Yahoo and their partner Web sites for phantom shoppers even as more resources are devoted to thwarting scammers.