The integrity of the criminal justice system is being placed in jeopardy because of a lack of funding for improvements in forensic science, a watchdog has warned.
Dr Gillian Tully, the Forensic Science Regulator, highlighted a "significant risk" of DNA contamination which could compromise evidence or mislead courts.
Her annual report revealed that an investigation has been launched into one case in which a complainant's sample could not be used.
Dr Tully said not all police forces are "fully committed" to reaching required standards, with some failing to recognise the impact of failures in the area.
She called for investment in the forensic systems currently used by police to ensure they can keep up with an increasing workload.
Forensic science carried out by instruction from defence lawyers was also said to be under significant financial pressure because of current legal aid funding.
Dr Tully called on forces and the Legal Aid Agency to make more money available.
"Police forces must not treat quality standards for forensic science as an optional extra - and neither must others delivering forensic science in the criminal justice system," she said.
"Progress has been made, but some forces say they can't afford to deliver both operational work and the required standards of forensic science.
"The standards are not an unachievable 'gold-plated' ideal - they are the minimum standards expected of any reliable forensic science.
"Funding for forensic science across the board, and particularly, perhaps, for defence provision via legal aid, must be at a level that enables the standards to be met.
"Otherwise we will face the costs, both in criminal justice terms and financially, of quality failures and loss of confidence in forensic science."
A number of "concerning" contamination-related issues in Sexual Assault Referral Centres - which provide support for alleged victims of rape and sexual assault - and police custody were raised to the regulator.
An investigation is ongoing following a case last year when contamination at a centre meant that a complainant's samples could not be used, as well as raising "serious questions" regarding cases examined previously.
Other issues included the same medical practitioner being asked to examine multiple suspects within a case, or even both the victim and the suspect within the same case.
The potential for DNA contamination in police custody remains a "significant risk" and interim guidance on the issue must be adopted "as a matter of urgency" to prevent contamination from compromising evidence or misleading enquiries or courts, the report said.
Full "elimination databases" to identify DNA contamination are not yet in place. "The risk of wasting a significant level of investigative resource on the basis of contaminating DNA profiles, or of the presence of an unknown DNA profile misleading a court are therefore not fully controlled," the report said.
There has been a rise in the number of issues referred to the regulator. In the year to November a total of 57 matters concerning quality were raised, including nine regarded as "high risk". This compared to 36 referrals in the previous 12 months, with seven classed as high-risk.
Elsewhere the report noted a risk of incorrect classification of some weapons and forecast that few forces will meet a deadline to reach required standards for digital forensics by this year.
Techniques including analysis of DNA, fingerprints and digital evidence play a major role in a range of criminal investigations.
The publicly-owned Forensic Science Service, which previously provided police forces with the majority of such work, was controversially closed in 2012.
Debbie Simpson, National Police Chiefs' Council lead for forensic science, said: "The police service is committed to achieving accreditation and improving standards of forensic science.
"This latest report of the forensic regulator has highlighted key priority areas for us to continue to enhance our response and prevent quality failures.
"We were successful in a bid to the Police Transformation Fund in order for the police to improve capability and standards. Existing expert networks across the country share best practice and offer immediate support when issues have been recognised.
"Nationally, we will continue to work in partnership with forces, forensic service providers and the forensic regulator to deliver the forensic strategy and respond to the challenges faced by the service."
Policing Minister Brandon Lewis said: "Controlling the quality of evidence is critical to reducing the risk of miscarriages of justice and criminal trials collapsing, as well as maintaining public confidence in the system.
"We fully support the timetable the regulator has set out. It is not optional and must be met by all organisations providing forensic services to the Criminal Justice System."
The Government has already launched a review of the internal governance of forensic science, he added.