Fake web traffic has long plagued the online publishing world, but Dr. Paul Barford, computer science professor at the University of Wisconsin, is claiming the problem might be worse than suspected. And it's costing some of the top online advertisers millions in wasted ad impressions.

Dr. Barford, who is also the chief scientist at startup MdotLabs, is slated to present a study at an Internet security symposium Wednesday in Washington, D.C., where he we will claim that 10 traffic networks are serving up more than 500 million invalid ad impressions a month.

"We estimate the cost to advertisers for this fraudulent traffic to be on the order of $180 million annually," he said in a statement in advance of the presentation.

Dr. Barford reached his conclusion by posing as a web publisher and signing up for several different traffic generation services, also called PPV networks, which he filtered through software that uses anomaly detection to identify fake website traffic.

The study comes as more publishers and advertisers are becoming aware of fake web traffic and taking steps to combat bots that are growing increasingly more sophisticated.

"We see bots playing games that we didn't see a few years ago," said Brian Pugh, a senior-VP of audience analytics at ComScore.

MdotLabs, the company that Dr. Barford co-founded this month, is among a number of firms that publishers, media agencies and advertisers use to identify bogus traffic. GroupM, for instance, employs the services of at least three such firms: Double Verify, Integral Ad Science and Spider.io. In February, the London-based Spider.io uncovered a cluster of more than 120,000 computers that had been infected by the Chameleon botnet, which was flooding websites with fake traffic.

Among the services firms such as MdotLabs offers is the ability for publishers to incorporate software onto their own sites for the purpose of weeding out fake traffic.

"From a publisher perspective, the platform allows them to differentiate themselves from lower-quality players and charge for higher-quality CPMs," said Timur Yarnall, the CEO and other co-founder of MdotLabs.

Estimates about the amount of overall fake web traffic varies. Mr. Yarnall claims that as much as 50% of all web traffic is fake -- which is likely on the high end.

ComScore has indicated that 36% of all traffic is non-human, though that includes certain bots -- such as those from Google -- which do not inflate ad impressions. The percentage has increased sharply from 2011, when it was just 6%. Meanwhile, nearly a quarter of visitors to so-called "long-tail sites," which have an overall reach of less than 1.5% of total internet users, are creating fake ad impressions, according to Mr. Pugh. He said the percentage of fake visitors to large websites is far less.