Bold claim: The Letby case hinges on questions about who should be trusted on the witness stand, and this story reveals a troubling gap between what juries hear and what regulators were investigating behind the scenes. But here’s where it gets controversial: the public narrative may have missed crucial context about a key prosecution witness whose fitness to practise was in doubt during the trial. In this rewritten account, you’ll find a clear, beginner-friendly rundown of what happened, why it mattered, and the broader implications for justice and medical accountability.
A doctor who provided central expert testimony on insulin poisoning for the prosecution in Lucy Letby’s case was under a fitness-to-practise investigation by the medical regulator at the time he testified. The General Medical Council (GMC) opened the investigation into Professor Peter Hindmarsh on the day he first gave evidence in Letby’s trial in late 2022, over serious concerns about whether he could safely practise. The inquiry was still active when Hindmarsh returned to the witness box for a second appearance a few months later. Great Ormond Street Hospital had referred Hindmarsh to the GMC after a formal probe led by his primary employer, the University College London Hospitals NHS Foundation Trust (UCLH).
Crucially, the jury hearing the Letby case was not informed about the GMC investigation into Hindmarsh, even though he was one of the prosecution’s key witnesses. While the GMC considered the matter, a medical tribunal had already imposed severe restrictions on Hindmarsh’s practice, stating there was a real risk he could endanger patients if allowed to practise without supervision. The tribunal also noted that the concerns could potentially affect his capacity to serve as an expert witness. Despite these warnings, Hindmarsh continued to provide expert testimony for the prosecution.
The Crown Prosecution Service (CPS) told the defence it would oppose any move to disclose the GMC investigation to the jury, arguing that the allegations had not reached a final decision. In the end, the GMC investigation was never resolved because Hindmarsh voluntarily erased himself from the GMC register, effectively ending the inquiry without a formal finding against him.
Guardian reporting later uncovered details of the allegations Hindmarsh faced at the time he testified. Letby, a nurse at the Countess of Chester Hospital, was convicted of murdering seven babies and attempting to murder seven others across trials in 2023 and 2024, and she received 15 life sentences. The Court of Appeal later denied her attempts to overturn the convictions. Letby has consistently maintained her innocence, and many supporters argue she was scapegoated for broader unit failures.
Since the convictions, a chorus of leading medical and scientific experts—both in the UK and internationally—argued that the prosecution’s medical case, including Hindmarsh’s testimony, was flawed. Notably, Dr Shoo Lee, a respected Canadian neonatologist, oversaw a 14-member expert panel whose findings challenged the prosecution’s position. The panel concluded that the babies died or deteriorated due to natural causes and substandard care, with no evidence of murders, insulin poisoning, or deliberate harm.
In February of the previous year, Letby’s lawyer sought a review by the Criminal Cases Review Commission (CCRC) to have the case reconsidered by an appeals court. The CCRC is examining the application, bolstered by the reports of 27 top experts, including members of Dr. Lee’s panel.
The notion of “multiple and wide-ranging concerns” about Hindmarsh’s practice is not new. Hindmarsh appeared twice as a principal prosecution witness during Letby’s first trial, which spanned from October 2022 to August 2023 and is widely regarded as one of Britain’s longest murder trials. Hindmarsh, a professor and consultant paediatric endocrinologist, brought extensive experience in treating children and had long associations with UCLH and Great Ormond Street hospitals.
He first testified on 25 November 2022, asserting that two babies—referred to in court as F and L—had been deliberately harmed via insulin poisoning. This underwriting of the insulin theory was central to the prosecution’s claim of attempted murder; the hypothesis was that insulin was injected into the babies’ fluid bags. Yet the jury was not told that Hindmarsh had been facing serious professional concerns in the months leading up to the trial. By mid-2022, his contract at Great Ormond Street had been terminated, though he remained connected to UCLH.
The hospital system declined to comment on whether the termination was related to the clinical concerns that were subsequently escalated to the GMC. Hindmarsh’s legal representatives at the time did not provide a public explanation either. When Hindmarsh was introduced in court, he described himself as a consultant at UCLH; the prosecutor’s line of questioning suggested a possible prior affiliation with Great Ormond Street, but the record left that ambiguity unresolved for the jury.
Before the trial, and in the weeks immediately preceding Hindmarsh’s testimony, the GMC and UCLH were already conducting investigations into his practice. On 15 November 2022, Great Ormond Street formally referred “multiple and wide-ranging concerns about Prof Hindmarsh’s practice” to the GMC, which opened its own formal inquiry ten days later. By late November and December 2022, Hindmarsh had become entangled in a growing regulatory cloud: he was informed about the GMC investigation, and UCLH reported further concerns and suspended him pending a return to work under tight supervision. The leaked material reviewed by the Guardian outlined serious issues, including alleged harm to patients, questionable diagnostic and treatment decisions, use of unusual therapies for children, and inadequate documentation.
In January 2023, a medical tribunal extended the restrictions, restricting Hindmarsh to work only at UCLH and under supervision. Despite these ongoing concerns, Hindmarsh sought permission to continue testifying as an expert witness in court. The tribunal noted that his general practice issues—especially around documentation and patient care—could undermine his credibility as an expert witness. The GMC did not object to his continued role, and the tribunal allowed him to testify, with the condition that he disclose to any instructing party that he was under GMC investigation.
Controversy extended to the question of disclosure. A source from the prosecution indicated the CPS did not learn of Hindmarsh’s UCLH-led inquiry until December 2022, raising questions about timing and transparency. The CPS argued against informing the jury about the GMC investigation, pointing to the lack of a final adjudication. Letby’s lead barrister at the time, Ben Myers KC, did not disclose publicly whether he sought to raise the matter with the judge, and Letby’s current representation has not commented on those decisions.
Hindmarsh returned to the stand for a second time on 24 February 2023. The jury remained unaware of the GMC investigation, the UCLH-led inquiry, or the constraints placed on Hindmarsh’s practice. His testimony reinforced the prosecution’s insulin-poisoning theory, which the prosecution argued was compelling evidence of deliberate harm, and which helped shape the trial’s broader conclusion about the safety and fate of the babies involved.
The insulin-related evidence was particularly notable because it rested largely on blood test results interpreted as indicators of insulin-induced hypoglycaemia in the affected babies, rather than on direct forensic proof of injections. Critics—many of whom now advocate for Letby’s innocence—argue that the blood tests used to detect insulin can yield unreliable results, and that Hindmarsh’s calculations and the overall narrative around insulin delivery via feed bags are questionable. They also emphasize that alternative explanations for the babies’ hypoglycaemia, including substandard medical care, were not given adequate weight in the final proceedings.
Overall, Hindmarsh’s role and the surrounding regulatory concerns have fueled ongoing debate about the Letby verdicts. Recent independent assessments, including a CCRC-supported review prompted by 27 expert opinions, have suggested questions about the medical basis of the convictions. In light of these developments, Hindmarsh’s status with the GMC was updated when he relinquished his registration on 14 November 2024. The GMC subsequently stated that Hindmarsh was no longer registered.
If you found this overview helpful, you might be wondering: should regulatory concerns about a witness’s fitness to practise influence the outcome of a trial, or should courts rely strictly on the evidence presented inside the courtroom? Do you think the Letby case highlights broader issues in medical accountability and jury transparency, or is it an exceptional situation? Share your thoughts in the comments and tell us which aspect you think deserves the most scrutiny in future high-profile trials.
