Commercial websites hosting child abuse images are remaining difficult to trace and hosting more severe content, according to the Internet Watch Foundation (IWF).

In its second annual report the charity, which works with the police and internet service providers (ISPs) to remove child sex abuse images from the internet, said the overall number of websites hosting child abuse images had fallen by 10 per cent.

However, three quarters of the existing sites were of a commercial nature and three quarters of these were registered with just 10 domain name registries. According to the IWF this makes it hard to trace them.

A representative for the IWF said: "These sites are not hosted in one country and images are loaded onto the sites through different servers. However they are hard to trace because these servers are moved between different countries every few days."

The IWF also found the severity of images had grown. It found just under two thirds of child sexual abuse websites that had been traced contained graphic images involving penetration or torture compared to just under half in 2007.

It found 69 per cent of the children appeared to be 10 years old or younger, 24 per cent six or under and four per cent two or under.

The IWF said some of these images were of the same children it had been seeing for the past few years.

"The images of these children are usually taken by paedophiles close to them and then put on their own personal websites. These then appear on commercial websites as they have been shared by different rings and spread," the representative said.

However, she was unable to specify why the severity of the images had got worse, but suggested that commercial websites used them on their home page to entice people to join.

To further combat the problem, the IWF suggests creating national notice and take-down schemes to remove criminal online content quickly, and sharing data, intelligence and tactics internationally to combat the cross-border nature of these crimes.