Rape of vulnerable women, especially those with learning difficulties, has effectively been “decriminalised”, according to a research academic employed by the country’s largest police force.

The claim is made by Professor Betsy Stanko, who is currently the Metropolitan Police’s assistant director of planning and has been researching the force’s investigation of rape for 10 years.

In a draft, unpublished report obtained by the Bureau of Investigative Journalism, Professor Stanko claims that despite a decade of reform the percentage of prosecutions and convictions of rape has remained consistently low. She says this is largely because two thirds of rape allegations drop out during the police investigation stage.

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The problem is particularly acute for people with vulnerabilities such as mental health issues and learning difficulties for whom the likelihood of getting their cases solved is extremely remote.

“These women face almost unsurmountable obstacles to justice,” Professor Stanko says. “Their rape is highly unlikely to carry a sanction, and in that sense, it is decriminalised.”

Her research shows that people with mental health issues are 40 per cent less likely to have their case referred to the police for prosecution than people without these difficulties. People with learning difficulties were 67 per cent less likely to have their case referred.

“Victim vulnerabilities effectively protect suspects from being perceived as credible rapists,” she says.

Professor Stanko calls for a change in the way rapes are investigated by the police, saying that anything other than a complete change is “tinkering around the edges”.

The call comes as the High Court ruled yesterday that the Metropolitan Police was liable to pay compensation for failing to properly investigate after two victims of convicted serial rapist John Worboys reported they had been attacked in 2003 and 2007 respectively.

Professor Stanko’s research, which is being presented at the LSE in two weeks’ time, delves into nearly 500 rape allegations made to the Met in April and May 2012. It indicates that little has changed for rape victims in the past 10 years.

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It shows that more than 80 per cent of people reporting rape to the Metropolitan Police are considered vulnerable to sexual attack for one of a range of reasons - including being under 18, having mental health issues or learning disabilities, having drunk alcohol or taken drugs prior to the attack and being in an intimate relationship with the suspect.

But rather than boosting the women’s credibility in the eyes of the police, these vulnerability factors actually mean their cases are substantially less likely to result in a suspect being charged.

“If a victim has mental health problems or is in a current relationship with the suspect, then the most likely outcome is that the case will be dropped,” Professor Stanko adds.

Her research also reveals alcohol consumption prior to the attack reduced the chances of referral to prosecutors by 45 per cent, as did a history of consensual sex with the suspect.

The new research has led Professor Stanko to call for a radical shake-up in the way rape is investigated by the country’s police forces.

She claims that a total cultural change is needed. Police investigators, she says, need to take a person’s vulnerability as evidence that they are more likely to be raped and investigate whether that vulnerability was exploited by the suspect.

Currently questions around consent rather than vulnerability dominate. “The debate, policies and law reforms continue to address issues of consent – the legal line separating sex from rape,” says Professor Stanko. “Exploitation is seldom recognised, though it is critical to how rape happens.”

Last night the Metropolitan Police declined to discuss Professor Stanko’s research.