An IT security firm has slammed 75 online slimming sites for engaging in misleading advertising and deceptive billing practices.

McAfee pointed out that, while diet scams have been around for a long time, their prevalence appears to be growing based on the ubiquity of the internet.

The firm claimed that as many as one-third of search engine results for keywords like 'hoodia pills' lead to misleading sites.

McAfee's SiteAdvisor researchers found that many diet sites engage in misleading "bait-and-switch" billing practices, where users are promised free samples but then automatically subscribed to an expensive monthly service.

In addition to suspect billing practices, McAfee also monitors criteria developed by the Federal Trade Commission on exaggerated product claims and results. 

Through the course of this analysis, McAfee gave the 75 sites a 'yellow' or cautionary rating.

"This problem has broad ramifications. Consumers search online for 'Hoodia', 'diet pills' and related key words an estimated 24 million times each month," said Kelly Ford, director of marketing at McAfee.

"Sadly, the bottom line is that most users who try these sites are more likely to lose money than pounds."