Child abuse 'big business online'
There are around 450 criminal gangs around the world making money from images of child sex abuse, the UK's Internet Watch Foundation has said.
The watchdog's annual report says that the 10 most prolific of these account for more than 650 web pages.
But despite these gangs being well-established online, the IWF says the the industry is not growing.
Such groups are, however, finding new ways to distribute images, the report adds.
Smaller social networks, image-sharing sites, free website hosting platforms and hacked websites are increasingly being used by content distributors desperate to avoid detection.
To help obscure their movements, they move their distribution networks regularly between different providers and countries.
Most of the gangs operate a pay-per-view system, charging a monthly fee of around £55 for access to images and videos.
Much of the material that becomes available commercially was originally traded privately between sex offenders.
The IWF, which enable people to report illegal content online via its website, estimates that about half of the child abuse images available online are commercial.
Publicly available websites hosting such content tend to be taken down very quickly but when one pops up, it illustrates the demand for such images.
One, hosted briefly in the UK in 2009, had over 25,000 visitors in the few days it was available.
Efforts to stay ahead of the gangs are difficult but they are being kept under control, the IWF said.
"Although internet usage and the volume of content continue to rise globally, we are not seeing a proportionate rise in commercial child sexual abuse material which instead appears to have remained fairly static over recent years," said Peter Robbins, chief executive of the IWF.
He puts that down to increased international co-operation.
To increase this effort, the IWF will for the first time tell international providers about illegal content it has identified.
It hopes that the tactics it employs will be mirrored in other countries.
"Nothing can be done on a national basis, we have to have every country adopting the tactics we use such as removing content at source, deregistering domain names and blocking content," said Ms Robertson.
The IWF received over 38,000 reports about illegal content in 2009. The majority of this was either hosted abroad or outside its remit.
Some 8,844 web pages were identified as containing images of child sexual abuse. Most were outside of the UK and were passed to one of the IWF's 35 sister organisations or to the relevant police authorities.
In the UK, the IWF issued 40 take-down notices on having images removed from the web.
Critics have accused the IWF of being ineffective, partly because it is separate from the police and partly because much of the illegal material online does not originate from the UK.
IWF spokeswoman Sarah Robertson admits that it is not a policing body but denies that means it cannot act.
"The amount of child abuse content hosted in the UK has fallen from 18% to 1% in the last seven years," she said.
"We don't feel we are fighting an impossible battle."
It is important to remember the serious nature of the content the IWF polices, she said, and that behind every image is a child being abused.
Some 44% of the content highlighted to the IWF last year depicted the rape of a child and 23% of it featured children below the age of six.