The undisclosed hospital probe involving expert witness Peter Hindmarsh has raised disclosure questions around the Lucy Letby trial and the handling of medical evidence.
Undisclosed Investigation Casts Shadow on Medical Evidence
Professor Peter Hindmarsh, a renowned pediatric endocrinologist and a central figure in the prosecution of Lucy Letby, gave evidence while under a formal medical investigation he failed to disclose to authorities. His failure to reveal the internal inquiry at University College London Hospitals (UCLH) occurred despite strict legal obligations requiring expert witnesses to declare any issues affecting their credibility. By March 11, 2026, Cheshire Police and the Crown Prosecution Service (CPS) stated they were never informed of the probe before Hindmarsh took the stand in November 2022. Disclosure rules in the British legal system demand absolute transparency from experts to ensure a fair trial for the accused. UCLH launched its internal investigation into serious concerns regarding the medical work of Hindmarsh prior to his testimony at Chester Crown Court. Internal documents suggest the inquiry was active during the period Hindmarsh served as a pivotal expert on insulin poisoning, a key pillar of the prosecution's case against Letby. The nurse was eventually convicted of murdering seven babies and attempting to murder seven others at the Countess of Chester Hospital. Expert witnesses hold a unique position of power in such proceedings, as their specialized knowledge often dictates the jury's understanding of complex biological data. Cheshire Police officials confirmed that the force remained unaware of the professor's professional standing issues until long after the initial verdicts. They maintained that the responsibility for disclosure rested with the witness. CPS representatives echoed this position, clarifying that no information about the UCLH inquiry reached their legal teams during the high-profile prosecution. This omission creates a significant legal hurdle for the Crown as the defense considers the implications for the integrity of the medical evidence provided. Credibility is the only currency an expert witness truly holds. Legal standards outlined in the Criminal Procedure Rules Part 19 require experts to provide the court with any information that might undermine their reliability. This specific failure to mention a hospital-led investigation into clinical work could be viewed as a breach of that duty. Hindmarsh was tasked with explaining the presence of synthetic insulin in two infants, Child F and Child L, arguing that the results proved intentional administration. His testimony was foundational to the prosecution's claim that Letby used insulin as a weapon on the neonatal ward. Should his own clinical judgment have been under review at the time, the defense would have possessed a powerful tool for cross-examination.
The Legal Framework of Medical Disclosure
Pediatric endocrinology is a narrow field, and Hindmarsh was considered one of its foremost practitioners. His expertise in glucose metabolism and insulin delivery systems made him the primary authority for the jury to trust. British law insists that any investigation by a professional body or employer must be disclosed if it relates to the expert's area of expertise. Such transparency prevents the jury from being swayed by individuals whose professional standing is in question. Failure to comply can lead to the evidence being discounted or, in extreme cases, provide grounds for an appeal against a conviction.
Defense lawyers often seek to discredit expert witnesses by highlighting professional failings or disciplinary actions. By remaining silent about the UCLH probe, Hindmarsh denied the defense the opportunity to probe whether the concerns raised by his employer reflected on his analysis in the Letby case. The trial lasted ten months and relied heavily on the interpretation of medical charts and lab results that few laypeople could decipher. Without the full picture of Hindmarsh's professional status, the jury's ability to weigh his evidence was arguably compromised.
Questions regarding the vetting process of medical experts have now intensified.
Police investigators typically rely on a witness's self-declaration of any investigations or disciplinary findings. No mechanism exists within the current system to automatically alert the police if a medical expert's employer begins an internal review. While the General Medical Council (GMC) tracks formal sanctions, internal hospital investigations often remain confidential unless they result in public disciplinary action. This gap in the information chain allowed Hindmarsh to continue as a key prosecution witness without his credibility being formally challenged in front of the jury.
Impact on the Insulin Poisoning Allegations
Hindmarsh focused specifically on the blood test results of Child F and Child L, both of whom survived their alleged poisoning. He argued that the high levels of insulin compared to low levels of C-peptide in their systems were biologically impossible without outside intervention. The defense struggled to counter his narrative, as Hindmarsh presented the data as definitive proof of criminal activity. His clinical work at UCLH involved the same types of complex hormonal analysis he performed for the Letby trial. If the hospital had doubts about his clinical accuracy or methods, those doubts were relevant to every sentence he spoke from the witness box; his silence on the matter is therefore a point of intense legal debate.
Evidence of insulin poisoning was a centerpiece of the trial because it was perceived as objective proof of foul play, unlike other cases that relied on more subjective observations of air embolisms. If the credibility of the primary insulin expert is damaged, the strength of those specific convictions may be re-evaluated. Several medical professionals and statisticians have already voiced concerns about the prosecution's methods in the Letby case. News of a hidden investigation into a lead witness only fuels the growing controversy surrounding the trial's outcome and the safety of the verdicts.
Legal experts suggest that the failure to disclose is more than a procedural oversight; it is a threat to the fundamental right to a fair trial. The Criminal Cases Review Commission (CCRC) often examines such nondisclosures when determining whether to refer a case back to the Court of Appeal. Previous cases in the United Kingdom, such as those involving the discredited evidence of Sir Roy Meadow, show how the downfall of a single expert can lead to the collapse of multiple high-profile convictions. Meadow's statistical errors in the Sally Clark trial eventually led to a major overhaul of how medical experts are used in court.
Cheshire Constabulary continues to stand by the prosecution, yet the silence of Professor Hindmarsh remains a complicating factor.
Records indicate that the UCLH investigation was not a minor administrative matter but a formal inquiry into clinical concerns. While Hindmarsh has not been found guilty of malpractice, the existence of the probe itself was a material fact the court deserved to know. Transparency is the bedrock of the expert witness system, yet the system appears to have failed in this instance. The CPS must now decide how to handle future cases involving Hindmarsh and whether his previous testimony in other trials must also be scrutinized.
Legal Consequences
Can the public truly trust a justice system that relies on the unvetted word of medical professionals whose own clinical performance is under official scrutiny? That nondisclosure strikes at the disclosure regime that underpins British law and at the credibility courts assign to specialist medical evidence. Expert witnesses are not supposed to be advocates for the prosecution; they are supposed to be objective servants of the court. When an expert cherry-picks what the court knows about their own professional standing, they effectively manipulate the scales of justice in a way that is nearly impossible to correct after a verdict is rendered.
The revelation that Professor Peter Hindmarsh withheld information about a hospital investigation is a catastrophic failure of the disclosure regime that underpins British law.
Such a situation highlights the dangerous arrogance of a medical establishment that views itself as above the standard rules of legal transparency. In the Lucy Letby trial, where the stakes were literally life and death for the reputation of the neonatal unit and the freedom of the accused, the omission of a clinical probe is inexcusable. We must move toward a system where the professional status of every medical expert is independently verified by the court rather than left to the honor system of the witness themselves. If the insulin evidence in the Letby case is eventually deemed unreliable due to the compromised credibility of its primary proponent, the British judiciary will face a reckoning that it has spent decades trying to avoid.