An investigation into alleged data manipulation at a forensics laboratory has identified more than 10,000 cases which "may have been affected", the National Police Chiefs' Council has said.
The NPCC said three-quarters of the cases, across 42 police forces, were traffic offences such as drug driving, with the rest including violent crime, sexual offences and unexplained deaths.
Retests have so far found no impact on cases of sexual offence cases, violence or homicide, the NPCC said.
But a number of retests had resulted in drug driving cases being discontinued and two road deaths had been referred to the Court of Appeal.
Two men have been arrested and five interviewed under caution by Greater Manchester Police over the alleged manipulation by individuals working at a Randox Testing Services site in Manchester
The alleged manipulation emerged earlier this year when a data anomaly in a drug driving case was reported to Randox.
The NPCC said retesting was either complete or under way for around 70 per cent 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, James Vaughan of the National Police Chiefs' Council 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.
Potential data manipulation at a separate facility, Trimega Laboratories, is also being investigated by Greater Manchester Police, the NPCC said.
This may affect child protection and family court cases.
It is understood the two suspects arrested in connection with the alleged malpractice also worked for Trimega.
Gillian Tully, of the Forensic Services Regulator, said all major forensic toxicology suppliers had been asked to carry out a detailed audit of a sample of their cases to ensure the issue was not more widespread.
The audits uncovered no such data manipulation.
She said: "If there was large-scale manipulation going on across the board I do expect it would have been found during that audit".
Although no murder and rape cases have yet to be found unreliable, some could be in a lower priority band for testing which have yet to be reanalysed, Mr Vaughan said.
Such cases would include finalised court cases which ended in acquittals or police investigations which had no further action.
Around 7,000 of these lower priority cases were road traffic and 2,000 were casework. It is expected the full retesting process will take two to three years to conclude.
Ms Tully added: "I'm not going to speculate on any motives because obviously there is an ongoing criminal investigation but we cannot just say it was a minor technical issue."