A loophole in extreme pornography laws allowed the likes of childkillers Mark Bridger and Stuart Hazell to get their hands on images portraying rape, campaigners have warned.
Rape Crisis South London and 100 other organisations have written to Prime Minister David Cameron calling for an urgent change to legislation to make it illegal to download pornography depicting rape.
And research has shown that of the top 50 accessible "rape porn" sites - found through a simple Google search - more than three-quarters advertise content portraying staged attacks on girls under 18.
It has been illegal to publish pornographic "portrayals of rape" in the UK since 1959, but pornographic rape material is legally available online as it is uploaded abroad and outside UK jurisdiction.
Extreme pornography laws, brought into force in 2008 after a campaign fought by the mother of a woman murdered by a porn-obsessive, aimed to close this loophole - when finalised, the legislation did not extend to rape depictions.
This has been tackled in Scotland, meaning anyone in possession of such images would be committing an offence and the Internet Watch Foundation could act to restrict or block access to sites.
Rape Crisis South London campaigner Fiona Elvines said:
"It is a serious omission not to have included images depicting rape and other non-consensual acts as they did in Scotland.
"This is not about making a distinction between real and simulated rape and child sexual abuse, with the latter being perfectly lawful to possess as long as it is 'fantasy' and actors are over 18.
"Permitting the possession of depictions of sexual violence as entertainment glorifies, trivialises and normalises such abuse - at a time when government statistics estimate that 85,000 women and girls are raped each year."
Bridger, who was jailed last month for murdering five-year-old April Jones, had cartoon pornographic images depicting bound and gagged youngsters being sexually abused, as well as images of apparently dead youngsters.
And Hazell, jailed for murdering 12-year-old Tia Sharp, had searched for vile child porn on the internet using terms such as ''violent forced rape'' and ''incest''.
Rape Crisis South London has conducted research that found of the top ten Google search results for 'free porn', half the websites host free rape pornography.
Sites include terms like "brutal rape", "real rape" and "savage rape" in their web addresses, the research found.
The letter to Cameron, also signed by Deputy Children's Commissioner Sue Berelowitz, urges him to consider reform in order that other Government work on tackling violence against women and girls is not undermined.
End Violence Against Women (EVAW) coalition director Holly Dustin said: "The government has promised to take action to prevent sexual and other violence before it happens and has taken some positive steps.
"However, if it is really serious about keeping its promise it must look at the cultural backdrop against which women and girls are abused. 'Rape porn' glorifies sexual violence and undermines the government's work.
"It sends a contradictory message about the seriousness with which sexual violence is taken."
Professor Clare McGlynn of Durham University, an expert in this area of law, said:
"The extreme pornography legislation is in urgent need of reform.
"The current law excludes the vast majority of pornographic images of rape. This is not a simplistic argument about rape pornography causing rape."
ends
A government spokeswoman said: "Rape is an abhorrent crime and that is why this government has driven forward significant progress in tackling violence against women and girls.
"We share the public's concern about the availability of harmful content on the internet and have already taken steps to ensure there are better online filters to protect children.
"But we want to look at what more can be done and so the Culture Secretary has invited internet providers to a summit this month. We will look closely at the issues raised in this letter."