The Home Office said many sexual abuse victims give up on seeking justice because they feel as though they are not believed or that they are the ones under investigation.
And going through the criminal justice process itself can be re-traumatising for victims and survivors, it added.
Data released by the Home Office has shown that of the 735 rape investigations closed by Sussex Police between April and September last year, 87 per cent were dropped due to problems gathering evidence.
It said in most of those cases, police said the alleged victim no longer supported the investigation.
It added that a suspect had already been identified in 67 per cent of them.
The charity Rape Crisis and the Criminal Bar Association said lengthy delays within the criminal justice system were to blame – a problem which, it said, had been exacerbated by the coronavirus pandemic.
“It wasn’t uncommon pre-pandemic for survivors to have to wait two years or more between reporting and their case reaching court,” said a spokesman.
“That is a very long time to effectively have to keep the memory of what might have been the most traumatic experience of a person’s life to date at the forefront of their thoughts.
“As well as this, the criminal justice process itself is too often re-traumatising for victims and survivors, who tell us they don’t always feel believed or even that they feel like they’re the ones under investigation rather than the suspect.”
Men, who are typically less likely to report rape, made far fewer allegations than women – 63 compared to 672 – and were less likely to drop their case.
The figures also show that just five per cent of rape cases closed by Sussex Police in that period resulted in a charge or summons.
Figures for the whole of England and Wales – excluding Greater Manchester – show that 42 per cent of rape investigations closed in the year to September 2020 were abandoned after those who reported attacks withdrew their support, while just 1.5 per cent resulted in a charge or summons.
In Sussex, 58 per cent of all rape cases closed over six months were dropped because victims did not support further action.
A Rape Crisis spokesman said it is common for people to withdraw from investigations.
Criminal Bar Association chairman James Mulholland QC called for a ‘reset’ of criminal justice resourcing, addding: “The principal reason why complainants in sexual allegation cases are walking away rests with a lack of prosecution resources and delays of years, which prevent the vast majority of such allegations progressing through to charge and trial.”
He said delays across the justice system had fuelled an ‘unacceptable’ rise in rape and other violent offences falling apart after allegations are made to police.
A Government spokesmann said it would work with forces to improve the investigation and prosecution of rape offences and do ‘all it can’ to restore faith in the justice system.
“We expect every report of rape to be treated seriously from the point of disclosure, every victim to be treated with dignity and every investigation and every prosecution to be conducted thoroughly and professionally,” they added.
In response, Detective Superintendent Jon Hull of the Sussex Police Public Protection Command, said rape investigations were among the most complex and sensitive investigations officers deal with.
And, in recognition of this, the force has changed the way it responds to such cases.
He said: “All rapes are investigated by specially trained detectives within our Safeguarding Investigation Units (SIUs), who also investigate child sex abuse offences and high risk stalking and domestic violence cases.
“In addition, dedicated Sexual Offences Investigation Team (SOIT) officers and staff lead and support all rape victims from the earliest stages of such investigations through to trial.”
Det Ch Supt Hull said an important element of Sussex’ Police’s drive to improve detections for rape offences is in ensuring investigations are of the highest quality.
He went on: “Sussex is part of a national pilot working with the Crown Prosecution Service (CPS).
“This involves working closely with CPS at a much earlier stage in the investigation with a view to speed up investigations.
“We also are working on a range of activity that is developing how we can reduce vulnerability, increase reporting and bring offenders to justice.”
Reports of rape and serious sexual offences continue to rise in Sussex, Det Ch Supt Hull said, adding that Sussex Police had increased the number of investigators working within SIUs to meet this demand, and continues to prioritise attracting, developing and retaining investigators to the force.
For more information about how Sussex Police responds to reports of sexual offences, and the support available to victims, visit www.sussex.police.uk/advice/advice-and-information/rsa/rape-and-sexual-assault.