By Charlotte Webster-
In a startling development that has reignited controversy around one of Britain’s most notorious criminal cases, it has emerged that a pivotal expert witness in the trial of nurse Lucy Letby was under a General Medical Council (GMC) fitness to practise investigation at the very time he was giving crucial evidence to the jury and the jury was never told. The disclosure has prompted legal commentators, medical professionals, and civil liberties campaigners alike to question whether one of the longest and most complex trials in recent UK history unfolded with all relevant information disclosed to the court.
Letby, a former neonatal nurse at the Countess of Chester Hospital, was convicted of murdering seven infants and attempting to murder seven others during her care between 2015 and 2016, in trials that concluded in 2023 and 2024. Her sentencing to whole‑life terms attracted widespread attention and raised deep debate over forensic science, medical evidence, and the role of expert witnesses in criminal proceedings. However, this latest revelation about one of the case’s key prosecution witnesses has thrust those debates back into the spotlight.
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At the heart of the controversy is Professor Peter Hindmarsh, a paediatric endocrinologist whose testimony on alleged insulin poisoning in two infants formed a cornerstone of the prosecution’s case.
Unbeknown to the jury, Hindmarsh was under investigation by the GMC for serious concerns about his clinical conduct, including allegations that he may have harmed patients and issues with his diagnosis and management of care.
The investigation was triggered by referrals from both Great Ormond Street Hospital and University College London Hospitals (UCLH) on the very day Hindmarsh first took the witness stand in November 2022.
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Despite this, and despite a medical tribunal voicing concern that the issues raised could “impact on his ability to act as an expert witness,” Hindmarsh was permitted to continue giving evidence at the trial. The tribunal placed restrictions on his clinical practice due to a perceived risk to the public, yet did not bar him from serving as an expert witness a decision that has since drawn intense scrutiny.
Prosecutors successfully opposed any attempt to inform the jury of his ongoing fitness to practise issues, arguing that because there was no final adjudication at the time, disclosure was not appropriate.
Hindmarsh later voluntarily relinquished his GMC registration in 2024, effectively ending the investigation before it reached any conclusion. This means the GMC made no formal finding against him, but it also meant the public, the defence and crucially the jury never had the full picture of the professional concerns that hung over such a central figure in the medical evidence presented.
The unfolding implications of this revelation have rippled far beyond the Letby case itself, prompting broader debates about how expert evidence is regulated and disclosed in criminal trials.
Legal analysts and campaigners argue this incident underscores systemic vulnerabilities in the way courts handle expert testimony a theme that has gained attention in other cases where medical evidence played a decisive role.
Last year, a former surgeon wrongly convicted in a separate case described expert witnesses as the “weakest link” in the justice system, highlighting the profound effect such testimony can have on life and liberty.
Critics of the way the Letby trial unfolded point out that the insulin poisoning theory, anchored in Hindmarsh’s testimony, was one of the most controversial aspects of the prosecution’s narrative.
Independent panels of neonatologists and scientists have since challenged the scientific basis for that theory, arguing that hypoglycaemia and other clinical events in the babies could have had natural causes or been the result of care deficiencies rather than deliberate intervention. These experts have called into question the reliability of the tests and interpretations on which the prosecution’s case depended.
Meanwhile, a series of documentaries and media explorations including critiques of how the case has been portrayed, such as The Investigation of Lucy Letby have amplified public discussion about whether the trial was conducted in a way that fully respected due process and the need for transparency.
Supporters of Letby argue that undisclosed issues like Hindmarsh’s GMC investigation underscore their belief that the conviction may have been unsafe, even as others insist that the verdict stands on solid ground.
The debate has also prompted wider reflection on the standards governing expert witnesses in criminal proceedings. In the UK, experts are obliged to disclose anything that might reasonably be thought capable of undermining their opinion or detracting from their credibility.
But it appears that Hindmarsh did not disclose the full extent of the concerns and the related investigations at the outset of his testimony, raising questions about compliance with those obligations and whether safeguards are adequate to protect the integrity of evidence presented in court.
More recently, additional criticisms have surfaced concerning other medical experts in the Letby trial, some of whom have faced scrutiny for their credentials or interpretations. In late 2025, another medical witness resigned after complaints were lodged by families affected by the case, underscoring ongoing tensions around who is deemed qualified to interpret complex clinical data in legal settings.
This situation echoes earlier contentious cases where expert testimony shaped outcomes later deemed questionable. In the early 2000s, for example, paediatrician Roy Meadow faced his own fitness to practise tribunal after his controversial evidence contributed to several wrongful convictions in child death cases, prompting reforms that now prevent convictions based solely on expert opinion.
With these threads converge, the Letby case stands not just as a dramatic criminal saga but as a focal point for discussions about criminal justice, forensic science, and the checks and balances that underpin verdicts in complex cases.
Legal commentators suggest that this latest twist could have implications for Letby’s ongoing application to the Criminal Cases Review Commission (CCRC), which is reviewing whether her convictions should be referred back to the Court of Appeal due to potential miscarriages of justice.
Proponents of that push argue that the failure to disclose critical information about an expert’s professional standing to a jury undermines the fairness of the trial. Detractors maintain that appellate courts require clear evidence of material impact before overturning verdicts, and that expert credibility, while vital, must be assessed in the context of the totality of the evidence.
Whatever view one takes, the intersection of medical regulation and criminal justice exposed by the Hindmarsh revelation has ignited discussions about whether reforms are needed from tighter oversight of expert witness qualifications to clearer disclosure rules and an independent register of experts used by courts.
In the near future, these debates may shape not only the final chapters of the Letby saga but the broader architecture of how courts weigh complex, specialist evidence in cases that hinge on life‑and‑death interpretations. For now, the wider legal and medical communities watch closely, aware that the ripples from this case extend far beyond a single trial.
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