Bold claim: a pivotal expert witness in the Lucy Letby case was under a fitness-to-practice investigation at the very time he provided crucial insulin-poisoning testimony. And yet the courtroom never learned of it. But here’s where it gets controversial: the uncovered details reveal a clash between regulatory scrutiny and the pursuit of a conviction, raising questions about fairness and disclosure in high-stakes trials.
A doctor who presented key expert evidence for the prosecution in Letby’s case—Professor Peter Hindmarsh—was investigated by the General Medical Council (GMC) for concerns about his fitness to practise, including allegations that he had harmed patients. The GMC opened its inquiry on the first day he testified, in late 2022, and the probe continued when he returned to testify three months later. Great Ormond Street Hospital had already referred Hindmarsh to the GMC after a formal review led by his primary employer, the University College London Hospitals NHS Foundation Trust (UCLH).
Crucially, the jury hearing Letby’s trial was not informed about Hindmarsh’s GMC investigation. During the period of investigation—and while Hindmarsh continued to provide expert testimony—the medical tribunal had already limited his practice, noting that he “may pose a real risk” to the public and that the concerns could affect his ability to act as an expert witness. Despite these restrictions, the tribunal allowed him to continue as an expert witness for the prosecution.
The Crown Prosecution Service (CPS) told the defence it would oppose any attempt to reveal the GMC investigation to the jury on the basis that the allegations had not reached a final adjudication. Ultimately, the GMC investigation was not concluded because Hindmarsh opted for voluntary erasure from the GMC register, effectively ending the inquiry without a regulatory ruling against him.
Guardian reporting later disclosed the specifics of the allegations Hindmarsh faced at the time of his evidence. Letby, a nurse at the Countess of Chester Hospital, was convicted in 2023 and 2024 of murdering seven babies and attempting to murder seven more, with a 15-life-terms prison sentence. Appeals failed, and Letby has maintained innocence. A substantial cohort of leading UK and international experts has since argued that the prosecution’s case, including Hindmarsh’s insulin-focused testimony, was flawed. Notably, Dr Shoo Lee—an esteemed Canadian neonatologist—supervised a 14-member expert panel whose February 2025 findings concluded there was no medical evidence supporting Letby’s conviction, including no evidence of insulin poisoning or deliberate harm; deaths were attributed to natural causes or poor care.
In February of the previous year, Letby’s lawyer, Mark McDonald, applied to the Criminal Cases Review Commission (CCRC) to refer her case back to the Court of Appeal for possible miscarriage of justice. The CCRC is reviewing that application, supported by the expert panel reports from 27 leading specialists, including Lee’s panel.
The Hindmarsh episode is summarized in terms of several layers of concern:
- Hindmarsh testified twice as a key prosecution witness during Letby’s first trial (October 2022–August 2023), part of what is believed to be Britain’s longest murder trial. He was a professor and consultant paediatric endocrinologist with decades of experience at UCLH and an honorary role at Great Ormond Street.
- He first testified on 25 November 2022, asserting that two babies, F and L, had been poisoned with insulin, implying deliberate harm by Letby.
- The jury did not hear that Hindmarsh was under GMC investigation and subject to early conditions that suggested potential risk to public safety. The UCLH-led process had terminated his contract with Great Ormond Street four months earlier, in July 2022, but the precise relation between those events and the later GMC action remained unclear to the jury.
- In the months before his second appearance, the GMC investigation progressed, and a formal tribunal in January 2023 renewed restrictions, keeping Hindmarsh under supervision at UCLH and limiting his clinical duties due to concerns about patient harm, documentation, and treatment decisions. Yet the tribunal permitted him to continue as an expert witness at court.
- Despite these concerns, the tribunal did not bar Hindmarsh from serving as an expert witness, and the GMC did not object to his continued testimony. He was required only to disclose the GMC investigation to any party instructing him, a standard obligation for expert witnesses.
Regarding the insulin-related evidence, Hindmarsh contended that test results and medical records showed the babies experienced hypoglycaemia (low blood sugar) due to insulin contamination of intravenous fluids, implying Letby’s direct involvement. The prosecution argued this was incontrovertible and could shape the jury’s view of other deaths. The judge’s summation allowed inference of deliberate harm in related cases if the jury found it proven in one, making the insulin evidence particularly influential.
Since the verdict, critics have challenged Hindmarsh’s insulin conclusions and the broader prosecutorial theory. Several experts who supported Letby’s post-conviction review have argued that the insulin testing methods could yield unreliable results and that Hindmarsh’s calculations may be flawed. They also point out that alternative explanations tied to poor care and natural complications were not adequately explored in court.
When the Guardian posed questions about Hindmarsh’s evidence, the GMC, and the UCLH-led probes, Hindmarsh’s team declined comment. The GMC later confirmed that Hindmarsh is no longer registered, having relinquished his registration on 14 November 2024.
What this suggests is a broader debate about how far to disclose ongoing regulatory concerns about expert witnesses in criminal trials, and how such disclosures might influence public trust in verdicts that hinge on specialized medical testimony. Do safeguards adequately balance fair contest in court with the public’s right to transparent professional accountability? How should courts handle testimony from witnesses who are under disciplinary review, especially when their conclusions are central to life-altering outcomes? Share your thoughts in the comments below to weigh the implications for future cases and judicial processes.
