Hundreds of cases could be reviewed after two men who work at a laboratory used by police to test drug samples were arrested.
Randox Testing Services (RTS) is used by forces across the UK to analyse samples used in prosecutions.
Police chiefs said it had been told 484 cases handled by the firm since November 2015 may have been affected.
The men, 47 and 31, were arrested on suspicion of perverting the course of justice and bailed, police said.
RTS said it was fully co-operating with the investigation and some tests could be “re-run”.
The allegations relate to drug tests analysed at its Manchester office.
‘No question mark’
In a statement, RTS said there was no evidence to say alcohol samples were affected because they were not tested at that site.
The firm, based in Northern Ireland, said the investigation centres on the “manipulation of quality control data, which supports test results”.
A statement added: “A number of toxicology results have been compromised. RTS are working tirelessly to fully assess the impact and implications for each case.
“Where possible, when viable, samples will be re-run to provide robust, uncompromised results.
“There is no question mark over the RTS quality system or the robustness of our practices and procedures.”
Chief Constable Debbie Simpson, the National Police Chiefs’ Council lead for forensic science, said Randox had provided each force with a list of cases that could have been affected.
“Working in partnership with the Crown Prosecution Service, we have provided guidance to forces so they are able to review each case to determine if compromised data played a part in prosecution and the CPS will then take appropriate action in any cases identified,” she added.