Scores of convictions, including rapes and murders, could be overturned amid allegations of data manipulation at a forensics lab.
It was previously thought that 484 cases were affected, but a police probe has found more than 6,000 toxicology samples could be at risk.
But around 90 per cent of the 6,000 samples can be retested, the National Police Chiefs' Council (NPCC) has said.
Two men have been arrested by Greater Manchester Police (GMP) over allegations that hundreds of cases were mishandled by Randox Testing Services (RTS).
A team of forensic experts are identifying any live cases which require retesting and past cases where convictions could be unsafe, the NPCC said.
"The majority of cases affected are Road Traffic Act offences such as drug driving.
"However, RTS provided toxicology tests for other offences including rape, assault and murder so it is possible these cases could be affected," a spokesman added.
Deputy Chief Constable James Vaughan, NPCC forensic expert, warned that the number of affected samples could change again as the investigation continues.
He said: "This is a serious breach of the very rigorous professional standards set by the Forensic Science Regulator for staff and organisations working in this critical field.
"We now have a clearer picture of the scale of this data manipulation and have been able to set out a plan of action in partnership with RTS, the Forensic Science Regulator and the CPS.
"The numbers affected could change as our investigations progress.
"We are prioritising the most serious and pressing cases but all cases where there could have been an impact on prosecution will be assessed, retested and appropriate action taken.
"It is important that we nationally prioritise retesting of samples to ensure that resubmitted samples do not flood the market and impact on other important ongoing cases.
"While there has been limited retesting to date, the evidence has shown that in the vast majority of cases, the original reporting was accurate. "
The Crown Prosecution Service has been warned that a number of cases due in court soon need retesting of samples.
The allegations came to light in January when concerns were raised about data from RTS in a drug driving case and a criminal investigation was launched by Greater Manchester Police.
Two members of staff at RTS Manchester laboratory, aged 47 and 31, were arrested and bailed on suspicion of perverting the course of justice.
It is possible that other police forces could be affected, as RTS were also sub-contractors to other providers, the NPCC said.
RTS's Manchester lab had its accreditation suspended on March 21 and has voluntarily suspended accreditation at its Northern Ireland site.
Law firm Freeman & Co previously suggests the case of a 26-year-old Chester man who was arrested on suspicion of drug-driving has been dropped as a result of the investigation.
The results claimed to show the driver was not only over the prescribed limit for cannabis, but his blood also tested positive for traces of cocaine and another drug, which the defendant disputed.