Bold takeaway: A key expert witness in Lucy Letby’s case faced a fitness-to-practise investigation during the trial, raising questions about how such concerns were handled and whether the jury should have known. But here’s where it gets controversial: the investigation never concluded, and Hindmarsh continued to testify, leaving many readers wondering about the safeguards surrounding expert evidence in high-stakes criminal cases.
A doctor who supplied crucial insulin-poisoning testimony for the prosecution in the Lucy Letby case was under scrutiny by the medical regulator at the time of his courtroom appearances due to serious questions about his fitness to practise. The General Medical Council (GMC) opened an inquiry into Professor Peter Hindmarsh over concerns that he had harmed patients as part of the allegations, coinciding with the early days of Letby’s trial in late 2022.
The GMC probe remained active when Hindmarsh testified a second time about three months later. Great Ormond Street Hospital subsequently reported Hindmarsh to the GMC after a formal review conducted by his primary employer, the University College London Health (UCLH) Hospitals NHS Foundation Trust. The jury in Letby’s trial, which resulted in a conviction for murdering babies and attempting to murder others at the Countess of Chester Hospital’s neonatal unit, was not informed of Hindmarsh’s ongoing investigation.
During the period of the GMC inquiry, and while Hindmarsh testified, a medical tribunal had already imposed severe restrictions on his practice, noting that he “may pose a real risk” to public safety and that the concerns could impact his ability to act as an expert witness. Despite these findings, the tribunal allowed him to continue giving evidentiary testimony for the prosecution. The Crown Prosecution Service (CPS) told Letby’s defence that it would oppose any move to inform the jury about the GMC probe because the allegations had not reached a final adjudication.
Ultimately, the GMC investigation did not yield a formal finding because Hindmarsh voluntarily removed himself from the GMC register, effectively ending the inquiry without a regulatory ruling against him.
The Guardian later uncovered details of Hindmarsh’s alleged misconduct at the time he testified, adding context to the broader controversy surrounding the case.
Letby was convicted in two trials—2023 and 2024—for murdering seven babies and attempting to murder seven others, receiving 15 life sentences. Court of Appeal judges denied her attempts to appeal. She has maintained her innocence, while supporters claim she was scapegoated amid broader concerns about care on the unit.
Since the convictions, many leading UK and international medical and scientific experts have challenged the prosecution’s medical case, including Hindmarsh’s insulin-related testimony. Notably, Dr Shoo Lee, a prominent Canadian neonatologist, and a 14-member expert panel concluded that the babies died from natural causes or care-related issues rather than deliberate harm or insulin poisoning. The panel emphasized a lack of medical evidence supporting the Letby convictions.
In February, Letby’s solicitor, Mark McDonald, petitioned the Criminal Cases Review Commission (CCRC) to refer her case back to the Court of Appeal. The CCRC is reviewing that application, with support from the expert panel’s findings and other independent reports.
Careful background: Hindmarsh appeared twice as a primary prosecution witness during Letby’s first trial, which spanned from October 2022 to August 2023 and is widely regarded as one of Britain’s lengthiest murder trials. Hindmarsh, a professor and consultant paediatric endocrinologist, had decades of clinical experience with children and affiliations with UCLH and Great Ormond Street as an honorary consultant.
His first appearance came on 25 November 2022, where he testified about two babies—referred to as F and L—alleged to have suffered insulin poisoning via contaminated feed bags, supporting the prosecution’s claim.
Jury transparency remained an issue: pre-trial concerns about Hindmarsh’s conduct were not disclosed to the jury. By July 2022 his contract with Great Ormond Street had been terminated, and he remained at UCLH under scrutiny. UK hospital representatives avoided comments about the link between his contract termination and the investigative findings, while Hindmarsh’s representatives declined to comment.
During his court appearance, Hindmarsh identified as a consultant for UCLH, and a prosecutor’s question about his status with Great Ormond Street led to a somewhat ambiguous exchange, which did not reveal the contract termination to the jury.
Crucially, the jury did not receive any information about the GMC proceedings, or about the UCLH-led inquiry and related restrictions on Hindmarsh’s practice. In November 2022, the GMC’s involvement began to surface publicly, followed by further developments in December and January, including reports of additional concerns and temporary suspensions.
Public documents and leaked materials described a broad range of alleged issues, including potential harm to patients, questionable diagnostic and treatment decisions, unusual use of treatments in vulnerable age groups, and questionable documentation.
By January 2023, a tribunal renewed restrictions, restricting Hindmarsh to work at UCLH under supervision. He later sought permission to continue acting as an expert witness, arguing that the restrictions didn’t relate to the specific realm of expert testimony.
Despite the ongoing concerns, the tribunal permitted Hindmarsh to continue serving as an expert witness for the prosecution, with the condition that any instruction to him would disclose his GMC investigation. This obligation aligned with criminal procedure rules requiring experts to disclose information that could undermine their credibility.
A December 2022 police disclosure confirmed Hindmarsh’s GMC investigation, but the CPS did not reveal the details to Letby’s defence until February 2023, just before Hindmarsh’s second appearance. The CPS maintained that the jury was not misled about Hindmarsh’s qualifications or experience.
Hindmarsh’s second testimony occurred on 24 February 2023, with the jury remaining largely uninformed about the GMC probe, the UCLH-led investigation, the practice restrictions, or the broader concerns at the hospitals.
Prosecutors presented Hindmarsh’s insulin-based evidence as a pivotal component of the case, characterizing the insulin findings as incontrovertible proof of deliberate harm. The trial’s summing-up allowed the jury to infer similar harm in other cases if evidence of deliberate action was established in one instance. The insulin cases helped drive the convictions, marking two of the three unanimous verdicts.
Yet the insulin-related conclusions have since faced skepticism. Critics—now including experts who have submitted reports to the CCRC—argue that Hindmarsh’s insulin measurements can yield unreliable results and that his calculations may be flawed. They also suggest alternative explanations for hypoglycemia that align with poor care rather than intentional poisoning, and that Hindmarsh did not sufficiently present those alternative possibilities to the jury.
The Guardian sought Hindmarsh’s perspective on the GMC and UCLH investigations, as well as whether he disclosed these matters before the trial; his spokesperson declined to comment. A GMC spokesperson stated that Hindmarsh is no longer registered with the GMC, having relinquished his registration on 14 November 2024.
