When Kiera was just nine years old she gave an interview on camera to Lancashire Police describing the harrowing details of the sexual abuse she had been subjected to over several years.

But a few months later, she says, police officers told her mother they had lost the recording.

“It was really hard, because I sat there for hours and hours telling people what had happened to me and for that to be lost, I just thought like what’s the point in doing it again?” said Kiera, now 19.

“They did want me to do it again, but I just couldn’t go through with it at the time.”

It wasn’t until nine years later, when Kiera was an adult, that she felt strong enough to provide her evidence again.

In October 2024, her perpetrators were jailed for almost 30 years for raping and sexually assaulting seven children, including her.

“It can really affect someone’s mental state. It’s also not protecting other people because these people then don’t get convicted of crimes.”

A Lancashire Police spokesperson apologised for the lost interview disc in her case, and said, since 2015, it had introduced new processes to prevent similar issues happening again.

Former police officers have told the BBC they are not surprised by the findings.

“It’s [evidence] chucked all over the place,” said one former officer.

“The amount of it is overwhelming… it’s unsurprising it gets lost or damaged,” another told the BBC.

Professor Carole McCartney, a criminologist and expert in evidence retention believes the loss of the dedicated Forensic Science Service (FSS) in 2012 is one of the reasons behind the growing proportion of cases affected by unavailable evidence.

Before 2012, all police forces could send exhibits that needed storing or analysing to the service, but the government-owned company was closed that year after making large losses.

Since then, police forces have had to make their own evidence storage arrangements and contract private providers for forensic services.

Prof McCartney said she had witnessed an officer pull out what he called a “box of horrors” from underneath a desk which contained various pieces of un-catalogued evidence including a plastic bag with a broken wine bottle in it and a car numberplate.

All Items held by the FSS from before 2012 were moved to a different facility – the National Forensics Archive just outside of Birmingham – that year, but that archive is for unsolved cases only and does not accept new items.

Exhibits in the archive were crucial in overturning the convictions of both Andrew Malkinson and Peter Sullivan.

Its director Alison Fendley says that without a dedicated forensic service, police forces were currently suffering from a lack of resources and expertise at a local level.

“Police forces have got lots of other things to do – archiving is not their day job and there’s so much material coming and going it must be difficult to keep on top of,” she says.

Meanwhile, backlogs at courts, the growth in online crime and the increase in digital evidence such as body worn video are all adding to a growth in the amount of exhibits police have to keep.

The National Police Chiefs’ Council (NPCC) said police and the CPS worked together to ensure evidence was “gathered and presented in a timely manner, bringing offenders to justice and ensuring victims are safeguarded”.

It said the data obtained by the BBC refers to all evidence that is either missing or unavailable when a defendant is going to trial after being charged.

And this could include situations where police cannot find an expert witness or may not be able to obtain a required medical statement.

A number of recent reports have raised serious concerns about police storage of evidence.

In 2022, His Majesty’s inspectorate of Constabulary and Fire and Rescue Services (HMICFRS) found police forces were “struggling to meet the demands placed on it by the digital age” as a result of the fast growth in digital evidence.

The University of Leicester’s study into police retention of investigative materials, found three quarters of lawyers it surveyed had worked on a criminal conviction where evidence had been lost, destroyed or contaminated. Almost half claimed this had happened on multiple occasions.

And Baroness Casey’s 2023 review into the culture of the Metropolitan Police found officers having to contend with “over-stuffed, dilapidated or broken fridges and freezers containing evidence including the rape kits of victims”.

It found an “overworked and inexperienced workforce” lacked the “infrastructure and specialism” for dealing with sexual offences, which existed before a specialist unit was disbanded in 2019.

The BBC study found around one in 20 prosecutions by the Met had been dropped as a result of missing evidence between 2020 and 2024.

By comparison around one in 50 were dropped across England and Wales.

The Met said the number included situations where police could not find an expert witness or were not able to obtain a required medical statement and to suggest it was simply down to lost evidence was misleading.

It acknowledged that on “a rare number of occasions” evidence is misplaced, adding: “We continue to make improvements to our recording systems to minimise this risk.”

The Home Office refused the BBC’s offer to comment.

The NPCC said: “When evidential issues occur in a case, the CPS will raise this with police for any action deemed necessary and we will work together to ensure these are resolved wherever possible.”

The results of a consultation by the Law Commission, which proposed re-establishing a national forensic service and making the mishandling of evidence a criminal offence in some circumstances, are set to go before Parliament next week.