A Guardian investigation has revealed a key prosecution expert witness was undergoing a professional investigation into his fitness to practice when he gave evidence at Lucy Letby’s trial.

Peter Hindmarsh stood as an expert witness in late 2022 and again in 2023 relating to the ‘insulin cases’, responding to the charges that Letby poisoned babies with insulin by injecting it into fluid bags. He told the court that he did believe there was evidence Letby had poisoned the babies at the Countess of Chester hospital.

The investigation of Hindmarsh by the General Medical Council found he may ‘pose a real risk’ to members of the public and said his actions ‘may have the potential to impact on his ability to act as an expert witness.’ The GMC never finished the investigation because Hindmarsh chose to voluntarily erase himself from the medical register. At the time that he gave evidence he was still working as a Consultant Paedriatic Endocrinologist at University College Hospital London, but had been sacked from his role as an Honourary Consultant at Great Ormond Street.

The role of prosecution experts in Letby’s trials has already come under considerable scrutiny, with a panel of fourteen world-renowned experts coming together last year to argue that the prosecution’s medical explanations for the murders, including Peter Hindmarsh’s evidence, had been misrepresented. They told a press conference that they had reviewed the cases in detail and found no evidence that any of the babies Letby has been found to have murdered, or attempted to murder, had been harmed by Letby or anyone else, and that there were alternative explanations for every death or collapse.

Letby’s case is currently being reviewed by the Criminal Cases Review Commission (CCRC), the only body who can now refer her case back to the Court of Appeal.