More than a dozen police forces have outsourced digital forensic investigative work to unaccredited private laboratories in the past year, at a time when a series of rape cases have been abandoned because of problems with digital evidence.

The collapse of four trials within two months because digital forensic evidence had not been shared with defence teams has shaken confidence in the criminal justice system and triggered a review of thousands of rape cases by the Crown Prosecution Service.

Now fresh concerns have emerged about the basic quality of digital forensic evidence being used to prosecute serious crimes, including rape and child abuse. One industry figure described the digital forensics market as a “race to the bottom”, saying police contracts had been awarded to the cheapest providers irrespective of whether they complied with minimum quality standards set by the government.

The Guardian has learned that:

At least 15 police forces, including Greater Manchester police and the Metropolitan police, have outsourced digital forensics work – typically the analysis of mobile phones and computers – to unaccredited private companies, some of which are subject to no regulatory oversight.

One private company that holds a major contract covering more than a dozen forces had its accreditation revoked last year after failing its first audit, but continued to perform forensic work for the prosecution.

Just 15 out of 43 police forces met a government deadline in October to bring their in-house laboratories in line with minimum quality standards for analysing mobile phone, computer and CCTV data.

Nick Baker, the national police lead on digital forensics and deputy chief constable of Staffordshire police, said that the deluge of mobile phone and computer data that police are now faced with has left some forces struggling to comply with quality standards. “The speed at which that’s come upon us is immense,” he said.

Some forces outsource all their work, while others do so when required to clear backlogs. However, concerns have been raised that some private providers are falling short of official standards.

Only a handful have been audited by the accreditation body, UKAS, to ensure that phone and computer data is being extracted in its entirety, that data is stored securely and that staff vetting procedures are robust.

One of the largest private providers, a company called Sytech based in Stoke, holds a contract covering more than a dozen forces, including Greater Manchester police.

In March last year, Sytech had its accreditation revoked after failing an inspection. It received a letter from UKAS, seen by the Guardian, stating: “Your organisation must cease immediately all related work done under the UKAS accreditation affected by this suspension.”

The company’s management promptly informed police forces of the suspension, but police continued to send forensics case work to the company.

Baker said his own force, Staffordshire police, had carried out a risk assessment and, as the suspension was not linked to technical procedures, there was deemed to be no additional risk. “This is not indicative of people not respecting quality standards,” he said. On a national level, he added that “nobody is trying to subvert the requirements”.

Gillian Tully, the government’s forensic science regulator, said there is “no excuse” for police forces to continue to use unaccredited providers. “It’s clearly of concern when contracts are being placed with providers that are not compliant,” she said.

The Guardian understands that Sytech failed its audit after an employee, who was sitting next to the inspector, opened a biohazard bag containing a phone without following a required safety protocol. The company had its accreditation re-instated last month.

The Met outsources digital forensics provision to a defence technology company, called Mass, which subcontracts casework to other private companies, some of which are unaccredited. City of London Police uses six external providers, three of which are not accredited.

Experts at five different companies told the Guardian they had serious concerns about the quality of digital evidence being admitted in courts.

One analyst, who specialised in defence work, said the prosecution sometimes “cherry-picked” text messages and images and did “as little work as possible up-front in the hope that [the defendant] pleads guilty”. Another analyst, from a different company, stated: “In many cases the prosecution evidence isn’t true.”

The head of a company that carries out prosecution work across the country, said laboratories were widely disregarding a requirement to disclose to courts if they were not accredited. “I would challenge you to find any report that does that,” he said. He added that, in his view, “99% of people who are charged are guilty as hell”.

In the past decade, digital forensics has evolved from a niche capability to a central element in the investigation of almost every serious crime. The electronic footprint left by suspects can help rapidly establish alibis, identify accomplices and expose spurious claims.

Yet if presented selectively such evidence can risk miscarriages of justice. In December, a man had his rape conviction overturned after four years, after deleted Facebook messages emerged that presented events in a different light.

In her annual report, the regulator raised concerns that so few police laboratories met the October deadline she set to gain accreditation for digital work. The Met and Greater Manchester police are among these. Tully said roughly half of all forces are set to miss a 2018 deadline for complying with quality standards on fingerprint analysis.

Others dispute whether the accreditation system, widely used to assess DNA and toxicology labs, is readily applicable to digital work.

“You’d be led to believe that there’s some magic in this accreditation and that if you’re not accredited you’re not good,” said Andrew Keogh, a criminal barrister based in Wigan and a visiting fellow at the University of Northumbria. “That’s not true at all.”

Norman Lamb, the chair of the science and technology select committee, which heard evidence from Tully this month, said the current tendering process for digital work left the sector vulnerable. “The whole fragility of the market and what happens to samples that are suddenly in insolvency, we can’t mess around with this, it goes to the heart of people’s rights as citizens,” he said.

Additional reporting by Sarah Marsh