Campaigners say they are prepared to take the Crown Prosecution Service (CPS) to court over claims sexual offences cases are being "dropped" without good reason.
A UK-wide coalition of women's organisations, represented by the Centre for Women's Justice (CWJ), said the CPS has covertly changed its policy and practice in relation to decision-making on rape cases, leading to a major fall in the number of such cases resulting in a criminal charge.
Government figures show there was a 23.1% drop in the number of rape cases taken on by the CPS in the 12 months to 2017/18, despite a 16% increase in police-recorded rape over the same period.
The End Violence Against Women coalition (EVAW) said this change in practice, and the lack of cases going to court, discriminates against women and girls, and is a major failure in protecting their human rights.
They say they have a dossier of more than 20 recent cases which were dropped by prosecutors for reasons they felt were spurious or unjustified.
This includes one victim who was raped at knifepoint and held prisoner for two days by her boyfriend, a man who was known by police to be violent.
Despite lots of evidence of violence against the woman, the CPS prosecutor dropped the case, saying WhatsApp messages she had sent to placate her attacker could be misinterpreted by the jury, campaigners said.
They are due to submit a "letter before action" to the CPS on Monday, urging them to review and change the way they handle serious sexual offence cases.
EVAW coalition co-director Sarah Green said: "We have strong evidence to show that CPS leaders have quietly changed their approach to decision-making in rape cases, switching from building cases based on their 'merits' back to second-guessing jury prejudices.
"This is extremely serious and is having a detrimental impact on women's access to justice."
She said: "We want the CPS to stop weeding out what they perceive to be 'weak' cases.
"Prosecutors should be judging cases on their merits - not on whether or not they think a jury will be prejudiced against the victim.
"We're not just a shouty group saying 'Let's have a go at the state'. But the refusal to listen and to implement what we say is a massive change for the way these cases are brought to court - we say, for the worse - is the reason why we have taken these measures."
Bonny Turner, who alleges she was raped, said her case did not even go to court despite her having messages which she says her alleged attacker apologises for his actions in.
Ms Turner, who has waived her right to anonymity, said she is now speaking out to encourage rape victims to report attacks to police and pursue justice.
"For myself, I am at the end of the road there is nothing else that can be done, he will never be prosecuted," she told ITV News.
"But I am speaking out for other women and I just feel like I have to do everything I can for myself.
"It's not something I can carry around with me and keep as a secret because I didn't do anything wrong.
"I tried to do the right thing by reporting it, I didn't want to let it slide because these sorts of things - the more they can just be let go without any consequences, the more the message goes to perpetrators and abusers that they can continue to do this sort of thing without consequence."
A CPS spokesman said cases were decided on a rigorous set of legal tests.
He said: "Sexual offences are some of the most complex cases we prosecute and we train our prosecutors to understand victim vulnerabilities and the impact of rape, as well as consent, myths and stereotypes.
"Decisions to prosecute are based on whether our legal tests are met - no other reason - and we always seek to prosecute where there is sufficient evidence to do so.
"Victims have the right to ask for a review of their case by another prosecutor, independent of the original decision-maker, and this is another way we can make sure we are fair and transparent in what we do."
Katie Russell, national spokeswoman for Rape Crisis England and Wales, said: "Through our frontline work at Rape Crisis, we know the reasons for this under-reporting are many and varied, but survivors tell us they include fear of being re-traumatised through what can be a very long and difficult process, with little prospect of seeing their perpetrator brought to justice at the end of it."