Tens of thousands of rape cases may have been dropped because of secret targets implemented by the Crown Prosecution Service, it has emerged.

The unofficial policy for prosecutors to have “levels of ambition” of achieving a 60% conviction rate in rape cases may have encouraged them to drop cases where there was not a high prospect of conviction.

The hidden benchmark could go some way towards explaining why rape prosecutions and convictions are currently at their lowest levels in more than a decade. Last year the Guardian reported prosecutors were encouraged by the CPS to take a more risk-averse approach in rape cases by taking a proportion of “weak cases out of the system”.

The CPS has repeatedly denied that it has changed its approach to rape cases. However, in a statement to the Law Society Gazette, which first uncovered the targets imposed on staff between 2016 and 2018, the CPS admitted they were “not appropriate” and may have acted as a “perverse incentive” deterring the prosecution of less straightforward cases.

Under the public code of practice a case ought to be prosecuted if it is in the public interest and the likelihood of a conviction is “realistic”, in other words greater than 50%.

However prosecutors were also asked to work to different “levels of ambition” targets. These performance measurements were introduced in 2008 for a range of crimes. The ambition levels were expressed as a percentage of the number of cases where charges were brought that resulted in conviction.

As of 2013-14, the conviction-rate target for violence against women and girls stood at 75%, covering domestic abuse and rape cases. In 2016 the target was disaggregated: the rape target was dropped to 60%, while the domestic abuse target rose to 76.5%.

Harriet Wistrich, the director of the Centre for Women’s Justice, said: “If you have a target and you want to try and meet that target … you’re essentially not applying the correct test. The test ought to be ‘more likely than not’ … They are in violation of their own code.”

The CPS denied the shift in “levels of ambition” for rape cases constituted a change in approach. It said the targets were referred to in its annual reports. However the Guardian could find no such reference.

The Law Society Gazette said they discovered the “levels of ambition” when they were referred to in HM Crown Prosecution Service Inspectorate reports.

The CPS is facing a judicial review challenge over other allegedly covert policy changes that are blamed for a dramatic collapse in the number of rape cases going to court.

While the number of rapes reported to the police nearly tripled between 2014 and 2018, the End Violence Against Women coalition (EVAW) points out that the number of cases charged and sent to court fell by 44%.

On Monday the Guardian reported on a leaked Cabinet Office report that said half of rape victims are dropping out of investigations, as a growing proportion do not want to pursue a prosecution even when a suspect has been identified.

Rebecca Hitchen, for EVAW, said: “This is a shocking admission from the CPS, as it acknowledges what we have repeatedly claimed – that performance targets linked to conviction rates will result in CPS prosecutors charging ‘stronger’ cases and dropping ‘weaker’ ones in order to make itself look better. This is why the numbers of rape cases reaching trial has plummeted and why we are judicial reviewing the CPS.”

A CPS spokesperson said: “Rape is a devastating crime, and CPS policy is clear: whenever the legal test is met, our dedicated prosecutors will bring charges, no matter how challenging the case. This policy has not changed. Every decision is, and always has been taken in the same way, following the Code for Crown Prosecutors.

“While it is important that we track trends, and constantly strive to improve performance, no individual charging decision is influenced by any factor other than the merits of the case.

“The growing gap between the number of rapes recorded, and the number of cases going to court is a cause of concern for all of us. We will implement any improvements suggested as part of the ongoing, cross-justice system rape review.”

• This article was amended on 15 November 2019. An earlier version said that for a period up to 2016, the CPS’s target conviction rate was 76.5% for domestic violence and rapes cases overall. This has been changed to 75%.