Sickest-of-the-sick child porn sites have quadrupled

In addition, nine out of ten victims of online child pornography are aged 12 or under.

The Internet Watch Foundation, which monitors web content, also warns that the number of sites showing the most severe types of abuse had risen from 7 per cent in 2003 to 29 per cent last year.

Its annual report says reports of child-sex images on the Internet rose by 34 per cent to 32,000. Of those, 10,656 web pages across 3,077 sites were confirmed as containing illegal child abuse content.

Nearly 60 per cent of all commercial child abuse websites sell child rape images, it added. Experts said all the findings were appalling, but added that the age of the victims was particularly alarming.

It is the first time such information has been made available. Some 90 per cent of the children being abused in the images reported by the public appeared to be under 12 and 80 per cent were female.

Foundation chief executive Peter Robbins said: "Sadly, we have to report new trends regarding the young age of the child victims in the images we assess and the dreadful severity of abuse they are suffering.

"These facts, coupled with the longevity of some commercial websites, mean the victims' abuse can be perpetuated for many years as the images are repeatedly viewed."

In 2004, 624 reported images showed children having penetrative sex - which the IWF categorises as Level Four of severity - and 117 depicted sadism or sex with animals - ranked as Level Five.

But last year there were 2,690 Level Four images reported and 352 Level Five images.

A number of sites have avoided closure by hopping servers across different legal jurisdictions, the IWF annual report said.

For example, one site has been reported to the foundation 224 times since 2002, it said.

By moving from country to country, they avoid being closed down permanently. Four out of five are based in the U.S. and Russia, with barely any in the UK.

Mr Robbins said: "The increase in reports processed by our hotline team is due in part to the increased awareness of our organisation but also to the identification of a growing trend in the use of remote image storage facilities.

"The UK has virtually eradicated the hosting of potentially illegal online child abuse content within its virtual borders."

Home Office Minister Vernon Coaker said: "The IWF annual report reveals deplorable trends in the sexual exploitation of children via new technologies."

While a general rise in Internet use is partly responsible for the increase in individuals detecting images of abuse, foreign police and governments are failing to clamp down, the foundation claims.

Britain has no authority to shut down sites registered with overseas companies. British users can still access them unless their service provider operates a barrier, which happens in 80 per cent of cases.