Pedophiles are increasingly hiding child abuse images on websites that appear to the casual internet user as ordinary web stores, according to the latest annual report by UK Charity, the Internet Watch Foundation.

The report (PDF) claims that the tactic is being used more frequently by child abusers, and allows them to view images by navigating to certain sections of the site. To visitors, the website looks completely legitimate, and raises no immediate suspicions.

"We received reports to our Hotline by online users who have stumbled across these sites. They pose challenges because when the website is accessed directly, only legal content appears," said Susie Hargreaves, chief executive of the IWF.

She continued, "it means that a commercial child sexual abuse business may be able to acquire legitimate business services if the website appears to host legal content when accessed directly - essentially tricking companies into providing their services for what is actually a criminal enterprise."

According to the report, those hosting the graphic content use web development techniques to disguise the website from the general public except for those that follow a certain digital path to access it. The method is akin to those used by retailers to make return visitors navigate to certain parts of the website.

While sites selling material of this nature declined during the last year, the new tactic of concealing child porn images on legitimate looking websites poses challenges for law enforcement agencies.

In the majority of cases the charity had been alerted by internet users about the collection of images after stumbling on them accidently. In the last year, the IWF had witnessed this concealment technique on 600 different websites, the highest number to date, and always on servers outside of the UK.

In the last two years the charity has located 998 unique sources of material. Of those identified only 440 were still active in 2011, and the report states that no new "top level" sources have been identified in the last year.

The exact method used in deploying these websites was not detailed as the IWF feels doing so would compromise existing law enforcement investigations.