Accessing online images of child sex abuse is quicker and cheaper than ever before, a watchdog warned yesterday.
Paedophiles peddling obscene material are taking advantage of free image sharing websites to reach customers, the Internet Watch Foundation said.
The proliferation of such sites, as well as blogs, message boards and social networking, has opened up new opportunities for the vile trade.
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 last year, a slight rise on 8,084 the previous year.
Its hotline team processed 38,173 reports overall, a 12 per cent increase on the number received the previous year.
The number of website addresses with obscene content has remained fairly stable for the last three years, but the number of domains has fallen considerably.
Almost three quarters (72 per cent) of child victims are aged 10 and younger and almost half (44 per cent) of images feature rape or sexual torture.
Many sites use automated programmes to move around different internet hosts, often at random in a bid to frustrate detection.
Others hack into legitimate commercial websites to use them as storage for obscene material after alerting customers to the address with spam email.
Investigators have identified 461 distinct "brands" selling digital child abuse images and video to customers worldwide.
The 10 most prolific brands accounted for 660 website addresses, with the largest controlling 102 website addresses alone.
The IWF also highlighted a new trend in which illegal websites separate their payment mechanism from the images and marketing.
The charity operates an online contact form that enables internet users to report potentially criminal material.
It then targets the material by serving takedown notices on hosting companies and compiling lists of suspect news groups and websites.
Staff also liaise with police forces and other agencies to remove content hosted abroad and trace those responsible.
Mr Robbins said information about recognised "gateway" websites must be shared internationally if police are to succeed in shutting them down.