The criminal convictions of 41 people have been quashed following an investigation into alleged data tampering at a forensics lab, the National Police Chiefs’ Council has said.

The NPCC’s forensics lead, chief constable James Vaughan, said the 41 convictions or guilty pleas, all relating to drug driving offences, had been reopened and overturned.

Another 50 drug-driving investigations had been dropped, with 2,700 results re-analysed since the allegations focused on the Manchester laboratory emerged last year.

Vaughan described the scandal as the “biggest breach of integrity in forensic science” in living memory. Some of the people affected are understood to be seeking compensation.

More than 10,500 criminal cases since 2014 have been identified as potentially affected following allegations that scientists at a Randox Testing Services site in Manchester had manipulated forensics data. There is an ongoing criminal investigation into the alleged tampering, with three former Randox laboratory staff now on police bail.

Vaughan said that 90% of the highest priority cases had now been retested. These included live criminal proceedings, convictions where the person was in prison, and violent and sexual offences. All the cases in which the outcome had been altered were drug-driving convictions.

Based on the samples retested so far Vaughan said the rate of flawed results appeared to be about 3% or lower. However, he added: “We shouldn’t underestimate the impact of an unsafe conviction on a driver. It is a significant issue for us in terms of undermining the confidence in forensic science.”

In some cases retesting process has proved impossible because some samples have been destroyed in line with storage regulations, or they have degraded over time. Cannabis, in particular, became less detectable in older samples, Vaughan said.

Randox is expected to cover most of the costs of retesting, with the final bill expected to be in excess of £2m. Vaughan said that all retesting was expected to be completed by the end of 2019.