- Video report by ITV News reporter Richard Pallot
More than 10,000 cases "may have been affected" by alleged data manipulation at a forensics laboratory in Manchester, an investigation has found.
Three-quarters of the cases, across 42 police forces, were traffic offences such as drug driving, the National Police Chiefs' Council (NPCC) said.
A number of retests have resulted in drug driving cases being discontinued and two road deaths have been referred to the Court of Appeal.
Although no murder or rape cases have so far been found unreliable, some could be in a lower priority band for testing which have yet to be reanalysed, James Vaughan of the NPCC said.
Such cases would include finalised court cases which ended in acquittals or police investigations which had no further action.
The alleged manipulation by individuals working at a Randox Testing Services forensics laboratory emerged earlier this year when a data anomaly in a drug driving case was reported to Randox.
Two men have been arrested and five interviewed under caution by Greater Manchester Police in connection with the alleged manipulation.
Potential data manipulation at a separate facility, Trimega Laboratories, is also being investigated by Greater Manchester Police, which could affect child protection and family court cases. It is understood the two suspects arrested in connection with the alleged malpractice also worked for Trimega.
The NPCC said retesting was either complete or under way for around 70% of the highest priority cases, with the rest expected to be completed by mid-2018.
Of the around 50 cases due to go to trial which have been dropped, some were discontinued due to there being no sample available for retesting, the sample was insufficient in quality or quantity to allow retesting or there had been degradation of evidence, Mr Vaughan said.
Not every court was sympathetic to requests for proceedings to be adjourned, leading to further cases to be ditched, he said.
He added that, to his knowledge, nobody had been held in custody as all the offences were summary.
Gillian Tully from the forensic services regulator said that in light of events all major forensic toxicology suppliers had been asked to carry out a detailed audit of a sample of their cases, but that no similar data manipulation had been uncovered.