The Crown Prosecution Service conducted a secret internal review that exposed its failings in rape cases – but failed to share it with inspectors who were conducting an official inquiry for a major government investigation into rape, the Guardian can reveal.

The CPS inspectorate and the government told the Guardian they were unaware of the existence of the report until January 2020.

A Guardian investigation has obtained a copy of the unpublished report, which examined 200 unprosecuted rape cases. It found that in the majority of cases prosecutors were overreaching by making “disproportionate” and “unnecessary” requests for additional information.

However, the Guardian has learned that the CPS did not share its findings with Her Majesty’s Crown Prosecution Service Inspectorate (HMCPSI), which was undertaking a review of CPS cases as part of a government review into falling rape prosecution rates.

The HMCPSI report was published in December 2019 and offered less stark conclusions than that found by the CPS’s own secret report.

In September last year, the director of public prosecutions, Max Hill, stated that there was a need for “a frank and full conversation” about the fall in the number of referrals, charges, prosecutions and convictions of rape. The CPS also stated it was a “partner in this review, and will take part honestly and openly to scrutinise and further improve our work”.

However, the CPS has been accused of a lack of transparency after the existence of the internal report was revealed.

Vera Baird, the victims’ commissioner for England and Wales, said: “The CPS has promised to be open and frank with those carrying out the government’s end-to-end rape review. So it is surprising that they did not disclose the very worrying results of this internal review at a time when the inspectorate was specifically examining the whole of the CPS role in rape cases.”

The internal CPS report found:

Prosecutors made disproportionate requests in more than half (56%) of administratively finalised cases examined.

Almost two-thirds (65%) of rape cases referred by police to the CPS for early investigative advice (EIA) had disproportionate requests for information.

Police failed to complete all actions asked for by prosecutors in more than three-quarters of cases (78%).

In all cases referred by the police for EIA, they were seeking a decision to drop “exceptionally weak cases”. This accounted for a third of all administratively finalised cases.

The inspectorate’s review found prosecutors made disproportionate requests in only 44% of administratively finalised cases. It also found that only 70% of action plans set by the CPS were not met with a proper response by police.

Responding to questions from the Guardian, the CPS said it had made HMCPSI aware of its internal report in “discussions” before the start of its inspection, but had only shared it at the inspectorate’s request in January. Asked if the HMCPSI was aware of the report when it was carrying out its inspection, the HMCPSI responded: “No.”

The internal CPS review, marked “official-sensitive”, found disproportionate requests included requests for “a weather report from 1972”, “school reports from the 70s and 80s” and care home records where there “was no indication that these could have or would be relevant”.

The review examined 200 cases that had been “administratively finalised” between October 2018 and March 2019. These are cases the CPS counts as “closed” where police have not responded to requests for further evidence. However, the cases could be reopened at a later date were new evidence provided. The proportion of administratively finalised cases increased from 22% to 29% of all pre-charge decisions made by the CPS between 2017-18 and 2018-19.

Other prosecutors had requested “all phone downloads” and “CCTV or 999 calls when it was clear there was no such evidence”. The report said the cases showed “some evidence that the exact nature of social media … was not understood by all prosecutors, for example the request for ‘downloads of all Snapchat communication’.”

In another case, a CPS prosecutor gave police an action plan with 30 actions “not specific to the case but a cut and paste set of generic actions”. These requests had the effect of killing the case, as police did not, or were not able to, meet the demand for information.

The report also reveals concerns about police actions. It found almost two-thirds of cases were of “poor quality”, which “indicates that the police are using the CPS as gatekeepers to quality control their files rather than undertaking proper supervision themselves”. The report also found police were “failing to respond whatsoever” to appropriate requests for more evidence in 75% of cases.

The CPS said the report found no evidence that disproportionate requests contributed to delays. It added that when requests were disproportionate, often they were necessary but prosecutors had failed to set clear parameters. All specialist rape and sexual assault prosecutors had received training to address the issues raised in the internal report, said a spokesperson.

Both the CPS and the HMCPSI said the reports had different aims, samples of cases and methodologies. The CPS said it “would not have been appropriate to share the report” and that “routine assurance work” was not “routinely circulated externally”.

In December, the HMCPSI concluded it had found no evidence the CPS was taking a more risk-averse approach – a claim not examined in the internal report – and charging easier cases to improve its prosecution rate. The CPS said the inspectorate report had provided an “independent affirmation of its charging decisions”. The report was fiercely criticised by victims’ groups, which accused it of betraying “a huge lack of curiosity” about the collapse in rape charging.

In a letter to the End Violence Against Women coalition seen by the Guardian, Kevin McGinty, the chief inspector of the HMCPSI, agreed the report was “incomplete” and said it had been expedited in order to inform the government review.

“We were unable to explore matters arising as we went on, or to cover ground we felt needed further work,” he wrote, adding that the inspectorate had recommended a further joint inspection with the police.

The findings of the internal report confirmed the experience of Rape Crisis’s independent sexual violence advocates, said Katie Russell, a campaigner within the Rape Crisis movement.

“What possible reason could the CPS have for effectively sitting on these findings and not even sharing them with the inspectorate?” she asked. “Why would they not want to fully and frankly engage with the ongoing end-to-end review into rape prosecutions which the government has ordered because there is widespread acknowledgement that the current system is failing? That is a question for them to answer.”

Falling prosecution and conviction rates, which are at a 10-year low, prompted the review of the criminal justice system’s treatment of rape. The number of rape charges have dropped by 51% since 2014, while reported rapes have more than doubled in the same period, from 20,751 to 58,657 in 2018-19.

A spokesperson for the attorney general’s office said: “Improving how the criminal justice system tackles rape and sexual violence is a priority of this government. A cross-government review into rape and serious sexual offence cases is under way, which will look at a wide range of issues within the system.”

The CPS is also facing a proposed legal case brought by the End Violence Against Women coalition represented by the Centre for Women’s Justice, who argue that the CPS has unofficially changed its policy and practice in charging rape and has become more risk-averse. A judge will decide on Tuesday whether to grant a judicial review against the CPS.