The Crown Prosecution Service (CPS) does not know how many rape cases have collapsed after defence claims of “sexsomnia” despite fears the rare medical condition is being exploited.

The prosecutions agency for England and Wales has previously pledged to “robustly challenge” sexsomnia claims after a string of high-profile cases where sexual offence suspects were acquitted – or had their investigations dropped – after the disorder was raised.

Suspects charged with serial sexual violence and child abuse are among those who have claimed they were in a sleepwalk state at the time of the alleged offending so could not be found guilty. Sleep experts say such claims can be difficult to disprove.

In one high-profile case, the CPS apologised and paid damages for failing to properly challenge a defendant’s claim that he had not raped a woman – and that, instead, she had been suffering from sexsomnia and engaged willingly while asleep.

Now a freedom of information request has revealed that despite its public promise to improve its hand­ling of sexsomnia claims, the CPS does not know how many cases it has dropped in the past five years, or refused to bring charges in, after sexsomnia was raised.

It also does not know how many times the defence has been used in court – and how many times it was successful, saying it did not “record or collate data” on defendants’ or suspects’ defences.

The CPS said all sexsomnia claims were “robustly interrogated” and suggested that collecting data on sexsomnia was not necessary because it undertook “detailed reviews” of individual cases to identify learning opportunities.

But Jade McCrossen-Nethercott, 32, who took legal action against the CPS after it dropped her rape case in 2020, said the lack of data collection was “absurd”. “If no records are kept, how can they identify patterns, track the misuse of this defence, or assess whether reforms are actually working?” she asked.

Unusually, in her case, the defendant’s legal team claimed that she – rather than her alleged attacker – had been suffering sexsomnia at the time of an alleged offence. The CPS later sent her a “lessons learned” letter, promising it had “taken steps” to prevent mistakes in future. But McCrossen-Nethercott said the lack of data collection “significantly undermined” the pledge. “Victims deserve more than well-meaning assurances. Without data, the CPS is essentially operating blindly,” she said.

Sexsomnia is a recognised psychiatric disorder that can cause sufferers to engage in sexual activity while in non-REM sleep.

They may have their eyes open but no awareness or memory of their behaviour. If a jury accepts a defendant’s claim that a person was in a state of “automatism” – such as sexsomnia – at the time of an alleged offence, they will be found not guilty.

Lab tests and partner histories can be used to try to establish whether a sexsomnia claim is genuine, but it is difficult to completely disprove. Many of those successfully pleading sexsomnia in court had no known history of sleepwalking and no formal diagnosis. Campaigners are calling for lab tests, bed-partner interviews, and medical history to be obtained as a minimum in all cases where the condition is raised.

In April an Observer investigation identified 80 cases over the past 30 years where defendants accused of rape, sexual assault or child sexual abuse claimed to have been sleepwalking or suffering from sex­somnia at the time, including 51 in the past decade and eight in the last year. The figures are likely a significant underestimate as many cases do not reach the public domain.

It also found evidence of law firms advertising the so-called “sexsomnia defence” and boasting of charges being dropped after it put pressure on the CPS behind the scenes.

The findings prompted a warning by Dr Neil Stanley, a sleep expert who has acted as a witness, that the defence was in some cases being used “cynically”. “I know in my heart of hearts there are cases where guilty men have got away with it,” he said.

The CPS said it robustly challenged all sexsomnia claims and pointed to three recent cases where it secured convictions.

Christopher Johnson, 40, was convicted in June of 16 sexual offences after a jury rejected his defence. In September, Connor Chambers, 31, was convicted of 19 sexual offences, including 11 counts of rape, after claiming to have sexsomnia. He was found guilty and jailed for 30 years, plus six on licence. Another man was jailed for 10 years in March for assaulting two university students.

A spokesperson said that in court “any defence of non-insane auto­matism is robustly interrogated in exactly the same way as any other defence”, adding: “Our prosecutors will always use the evidence available to challenge defence claims.”

They added that the CPS had introduced safeguards in sexsomnia cases, including referring decisions to drop cases to senior officials for approval. But when asked how many such referrals had been made, it said it did not know and that it would take too long to manually review.

Victim Support called for the CPS to reform its approach.

Michaela-Clare Addison, national sexual violence lead at the charity, warned that it could become the new “rough sex” defence, a colloquial term for defence claims that victims consented to being seriously hurt – and sometimes killed – during rough sex that went wrong.

As well as tracking the prevalence and outcome of cases where sexsomnia was raised, she said the CPS should monitor whether it arose before or after the defence firm became involved.

She added: “It is vital that the CPS collects data on sexsomnia cases so it can identify and tackle this growing trend.”