The Internet Watch Foundation (IWF) said 29% of the web pages reported to it last year showed children being raped or youngsters forced to suffer sadism or bestiality.
It compared with just 7% in 2003.
The IWF, which is funded by the telecommunications industry, also showed the number of reports of child sex images on the internet was up 34% overall to just under 32,000.
Of those, the group found 10,656 web pages across 3,077 sites contained illegal child abuse content.
Nearly 60% of all commercial child abuse websites now sell child rape images, it added.
For the first time, it revealed that more than nine out of 10 victims in the vile images reported by the public appeared to be under 12 years of age, and 80% were female.
The figures appeared to show the severity of online child abuse content is increasing.
In 2004, 624 reports showed children having penetrative sex — which the IWF categorises as level four of severity — and 117 depicted sadism or bestiality — ranked as level five.
But last year there were 2,690 level four web pages and 352 level five sites.
A number of illegal 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 IWF 224 times since 2002, it said.
IWF 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.
“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.”
He added: “The UK has virtually eradicated the hosting of potentially illegal online child abuse content within its virtual borders.”
More than 80% of child sex abuse sites are hosted in the US and Russia, up from 67% last year.
Home Office Minister Vernon Coaker said: “The IWF annual report reveals deplorable trends in the sexual exploitation of children via new technologies but behind these figures there is a UK success story.
“The IWF has made dramatic and continued progress in tackling the availability of illegal images of child abuse and has made a significant and on-going contribution to the eradication of exploitation sites and the prevention of access to sites hosted abroad.”