A forensic scientist has been suspended from their role at the Metropolitan Police and an "urgent" review has been launched into more than 30 cases of apparently botched examinations.
Scotland Yard says a member of its Forensic Services staff allegedly failed to carry out tests and lied to investigators about the work being undertaken and its progression.
Twenty-one sex attacks are being urgently reviewed, while the other cases from between 2012 and 2017 include violent crime, drug cases and burglary.
The force said in a statement: "We are urgently conducting a review to understand whether there is any risk to the criminal justice process and to take remedial action where necessary.
"All victims in the affected cases have been contacted, where it has been deemed appropriate to do so."
The suspended scientist has been audited and the Met said it is "satisfied that there are no other instances of undeclared casework".
The case comes amid a national crisis in forensic services, following the closure of the publicly owned Forensic Science Service in 2012.
Techniques including analysis of DNA, fingerprints and digital evidence play a major role in a range of criminal investigations and the move forced police forces to either bring the services in-house or use private providers.
One, Key Forensic Services, collapsed in January, potentially affecting thousands of cases.
Another, Randox Testing Services, was hit by claims of manipulation of results.
Around 10,000 cases - three-quarters of which were drug-driving and the rest violent crime, sexual offences and unexplained deaths - were affected across 42 police forces.