Laws controlling pornography in England and Wales need to be strengthened to make it illegal to download staged rape images, campaigners say.
In a letter to PM David Cameron they say sexual violence as "entertainment" causes a "huge cultural harm".
The deputy children's commissioner for England is among those calling on the government to close the "loophole".
But the Ministry of Justice recently said there is "no evidence" that such material harms society, it has emerged.
Staged rape scenes, which do not break any laws as they feature actors aged over 18, are outlawed in Scotland.
The campaigners say legislation introduced in 2008 was intended to make it an offence to be in possession of "extreme" pornography" but did not cover most simulated images of rapes or attacks.
The letter to Mr Cameron has been signed by Deputy Children's Commissioner Sue Berelowitz and more than 100 other academics, experts and campaign groups.
It says the government should consider reform so that its work on tackling violence against women and girls is not undermined.
The campaigners say current legislation sends a "contradictory message about the seriousness with which sexual violence is taken".
It "allows the lawful possession of pornographic images depicting rape which promote sexual abuse of women and girls in England and Wales", they add.
They also point out that the recent trials of childkillers Mark Bridger and Stuart Hazell both heard they had viewed "violent and misogynistic" images.
But a letter from a Ministry of Justice official has emerged in which campaigners were told that claims of a link between violent rape depictions and actual crimes are not enough to justify a change in the law.
Dave Pearson from the ministry's criminal policy unit wrote in early May: "We have no evidence to show that the creation of staged rape images involves any harm to the participants or causes harm to society at large."
He said the Internet Watch Foundation had investigated real and depicted images of rape and believed "all the content they examined to be staged".
The letter drew a distinction between films portraying actual rape and those where "high quality technical production values" make it obvious that they are staged.
Mr Pearson acknowledged campaigners' concerns about staged rape "potentially glorifying sexual violence and somehow encouraging a failure to take such violence seriously".
He added: "However, we are not convinced that evidence for this is any more than anecdotal and cannot, therefore, be used to justify a change in the law."
Professor Clare McGlynn from Durham University, who received the letter, told the BBC: "I was surprised at the Ministry of Justice position and thought it was regrettable.
"It goes against the spirit of measures already in place in this country and also against the thinking of the current and previous governments about the need to take steps to prevent rape.
"It should be quite straightforward to change the law and show that we don't think it's acceptable to get sexual excitement from pornographic images of rape."
It has been illegal to publish "portrayals of rape" in the UK since 1959, but material can be legally available to download online if on foreign websites.
The campaigners are calling for the Criminal Justice and Immigration Act 2008 to be amended. It introduced restrictions on "extreme pornography" following a campaign by Liz Longhurst, the mother of Jane Longhurst, who was murdered by a man obsessed with violent websites.
Support centre Rape Crisis South London said its research found websites featuring the staged material "explicitly advertise sexually violent content" including simulations of attacks on schoolgirls.
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 ."
A Ministry of Justice spokesperson 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 [Maria Miller] has invited internet providers to a summit this month. We will look closely at the issues raised in this letter."