Accessing online images of child sex abuse is quicker and cheaper than ever before, a watchdog has warned.
Paedophiles peddling obscene material are taking advantage of free image-sharing websites to reach customers, the Internet Watch Foundation (IWF) said.
The proliferation of such sites, as well as blogs, message boards and social networking, has opened up new opportunities for the vile trade, it said.
The IWF said legitimate free online services are "vulnerable" to exploitation through the storage, sale and sharing of illegal images.
As a result the true scale of the underground industry remains almost impossible to measure as the number of complaints to the IWF continues to rise.
Peter Robbins, who heads the IWF, said a "persistent core" of sites are using increasingly sophisticated techniques to evade police. He said: "The techniques used by criminals who buy, sell, share or collect child sexual abuse images are sophisticated and are diversifying.
"Over half of the material we deal with is related to commercial payment mechanisms confirming an ongoing demand for images of children being sexually abused.
"The scale of all child sexual abuse content online is difficult to measure because the methods of operation are changing, becoming quicker, cheaper and more opportunistic than ever before.
"The distributors are increasingly using legitimate internet services to make the images available, from free hosting platforms and image sharing websites to social networking areas and hacked websites."
The watchdog charity reported 8,844 instances of child sexual abuse content to relevant authorities in 2009, a slight rise on 8,084 the previous year. Its hotline team processed 38,173 reports overall, a 12% increase on the number received the previous year.
