By Rory Cellan-Jones
BBC News technology correspondent
The IWF was set up in 1996 to police access to images of child abuse.
The BBC goes behind the scenes at the Internet Watch Foundation to see how its researchers cope with the psychologically demanding job of policing sites peddling images of child abuse.
The watchdog that blocked a Wikipedia page last year over a rock album cover says it still believes that the image at the heart of that controversy was illegal.
But the Internet Watch Foundation (IWF) says it has moved on after the row in December 2008, and claims its quest to eradicate child abuse images from the web is now having real success.
Critics have accused the organisation of being both unaccountable and ineffective. This week the BBC was given unique access to the work of the IWF, as the watchdog tries to fight back.
It's in a house in a quiet village north of Cambridge that the Internet Watch Foundation runs a hotline for reports of child abuse images. It's a job the self-regulatory body was given in 1996, as the internet industry sought to avoid direct control by the police or government.
Upstairs at its HQ there is a door marked "IWF staff only - image viewing in progress". I was allowed in, to find four analysts at work, sifting through reports that have arrived overnight from members of the public.
The analysts, who mostly have backgrounds in IT, prefer to remain anonymous, but one of them, Karen, agreed to tell me about her work.
She showed me a list of the reports she had to deal with that day, some from members of the public who said they had been looking for adult sites but had been shocked to come across child abuse images.
Some callers fear they may be accused of downloading illegal images.
Her first task is to try to determine whether the images are in fact illegal under UK law. "My next step would be to chase that image or website to the country where it is located," she said.
In almost all cases, the offending sites are abroad, and Karen contacts one of the IWF's 35 sister organisations - if there is one in the country concerned - and informs the UK police.
I put it to Karen that it must be difficult to do such emotionally draining work, especially when some feel that it's not even worthwhile. "There are times when I questioned my own sanity for doing this," she admitted.
"However, I'm a mother with two children, I feel that it's an important job and someone needs to do it. That's how I get through the day."
Karen sees some sites cropping up time and again, with the criminals moving them from country to country, trying to stay one step ahead.
She said: "We're also finding a trend towards sexual abuse images of younger and younger children, and more severe forms of abuse."
She explains that most of the sites operate a pay-per-view system, charging $80-100 (£55-68) per month for access to images and videos.
The offending sites are also added to the blacklist compiled by the IWF for internet service providers, which then block access.
There are times when I questioned my own sanity for doing this
Karen, IWF researcher
It was this process which led to the blocking of a Wikipedia page about a 1970s album cover featuring an image of a naked young girl. That ruling, reversed within days, threw a spotlight on the IWF, and the transparency of its procedures.
David Gerard of the Wikimedia Foundation, which runs Wikipedia and other similar projects, says the incident showed the IWF to be "ham-fisted and incompetent at every level".
He maintains that there is no way to block illegal content "without massive collateral damage to speech, communication, learning and society".
Dr Richard Clayton , a computer scientist at Cambridge University, has a different criticism - that the watchdog suffers from confusion about its aims.
His research shows that, while banks manage to get phishing sites taken down within hours, the IWF takes an average of 28 days to get child abuse sites removed. "'Are they trying to get sites removed from the internet?' he asks, if so they're doing a poor job.
"Alternatively, if they think their aim is to catch the criminals putting up this material, then they should become part of the police."
The IWF censored an image that appeared on a Wikipedia page
Peter Robbins, chief executive of the IWF, says the various critics are ignoring the fact that the watchdog's mission is proving a success.
He says the IWF has learned from the Wikipedia controversy, though he still believes the image on the album cover was illegal. "Was that an image of a pre-pubescent girl? Yes. In our view it's not OK," he says.
But he accepts that it was not realistic to ban an image that was widely available in shops and on the internet. "I don't want to dwell on this incident. The fact is that we are dealing with some horrendous content on a daily basis and we're working really hard with hotlines and law enforcement bodies to try to stop the distributors of these images."
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