Letby trial expert GMC probe raises questions over crucial insulin evidence

Published: 03 March 2026. The English Chronicle Desk. The English Chronicle Online.

In a deeply troubling revelation that has shocked legal experts and medical commentators across the UK, it has emerged that a key expert witness in the trial of Lucy Letby faced a GMC investigation into his fitness to practise at the very time he gave crucial evidence about alleged insulin poisoning. This startling development has intensified debate over the safety of Letby’s convictions.

The focus of this renewed scrutiny is Professor Peter Hindmarsh, a respected paediatric endocrinologist whose testimony was central to the prosecution’s claim that two infants had been deliberately poisoned with insulin. Court records now show that on the very day Hindmarsh first took the stand in late 2022, the General Medical Council launched a formal probe into his clinical practice, prompted by concerns raised by Great Ormond Street Hospital and University College London Hospitals. Yet the jury was never made aware of this investigation.

According to investigative reporting, the GMC inquiry targeted serious concerns about Hindmarsh’s practice, including alleged patient harm and questionable clinical decisions. Documents seen by reporters detailed accusations involving inappropriate treatments and inadequate record‑keeping, raising questions about his ability to offer unbiased expert testimony. Despite these issues, Hindmarsh continued to serve as a prosecution witness.

The revelation has ignited intense controversy because Hindmarsh’s evidence underpinned two of the prosecution’s most impactful claims against Letby. His interpretation of blood tests, which he attributed to insulin introduced into intravenous fluid bags, was presented in court as strong scientific proof of deliberate wrongdoing. Those interpretations formed a compelling part of the case that led to unanimous guilty verdicts in respect of two infants.

The Crown Prosecution Service has defended its conduct, stressing that the allegations against Hindmarsh had not been finally adjudicated and that the jury was not misled as to his qualifications. Yet, critics argue that withholding information about an ongoing GMC investigation deprived the defence of context that could have challenged the weight and credibility of his testimony.

What is clear from the record is that during the period Hindmarsh gave evidence, a medical tribunal had imposed severe restrictions on his clinical practice, concluding that he “may pose a real risk” to patients. These restrictions, on their own, prompted debate at the time over whether he should be permitted to act as an expert witness, especially given that the tribunal itself noted that the issues under inquiry “may have the potential to impact on his ability to act as an expert witness.” Nevertheless, the tribunal allowed him to continue in his court role, requiring only that he disclose the ongoing investigation to those instructing him.

In a twist that has further fueled uncertainty, Hindmarsh ultimately removed himself from the GMC register in late 2024 through a process known as voluntary erasure, effectively terminating the fitness to practise investigation before its conclusion. As a result, there was never any regulatory finding against him and no formal conclusion to the serious concerns that had been raised.

This issue has become deeply intertwined with broader criticisms of the medical evidence presented at Letby’s trial, which ended in her conviction for seven murders and seven attempted murders. While those convictions were upheld at the Court of Appeal, Letby and her supporters have consistently argued that the prosecution’s scientific case was flawed. In particular, international teams of medical experts have contested the insulin poisoning theory at the heart of Hindmarsh’s testimony, arguing that the blood tests he relied on are known to produce unreliable results.

One such expert, Canadian neonatologist Dr Shoo Lee, led a panel of 14 specialists who concluded that the babies’ collapses and deaths were more plausibly due to natural causes or care failings at the Countess of Chester Hospital’s neonatal unit. That report has been submitted to the Criminal Cases Review Commission, which is currently assessing Letby’s case for possible referral back to the Court of Appeal.

In interviews and public statements, critics of the prosecution’s medical evidence have been adamant that Hindmarsh’s opinions were not only contested but fundamentally unsound. They say his calculations were incorrect, that he overstated the reliability of the diagnostic tests, and that he failed to present alternative explanations for the infants’ conditions that might have supported the defence’s argument of systemic failings rather than deliberate harm.

This debate has been amplified by supporters of Letby, many of whom argue that the prosecution’s case relied too heavily on contested medical theories and not enough on incontrovertible factual evidence. They point to the absence of direct forensic proof linking Letby to any intentional act of poisoning, noting that much of the case was circumstantial and hinged on contested expert interpretation.

The developments have also raised questions about legal disclosure rules and the process by which expert witnesses are vetted and presented in court. Under criminal procedure rules, expert witnesses are obligated to disclose anything that might reasonably undermine their opinions or detract from their credibility. Observers have debated whether Hindmarsh met that obligation, given the timing of the GMC investigation and the fact that the defence was reportedly not informed until shortly before his second court appearance.

For families of the victims, these revelations are profoundly distressing. Many of them sat through the trial believing that the scientific evidence presented was unassailable. Others have expressed dismay that the full context of Hindmarsh’s professional situation was not shared with the jury. For them, the stakes are deeply personal, and the ongoing review by the CCRC represents one of the few avenues left to challenge the convictions.

For legal scholars and medical ethicists, the case has become a case study in the complex interplay between scientific expertise and the justice system. The questions raised about expert credibility, disclosure obligations, and regulatory oversight go beyond any single trial, touching on fundamental issues about how courts rely on specialist knowledge and how that knowledge is vetted and communicated to juries.

As the CCRC continues its review and as international experts weigh in, the spotlight on the Letby case shows no sign of dimming. Whether these developments will prompt a fresh appeal or further inquiry remains to be seen, but the revelations about Hindmarsh’s GMC investigation have already reshaped public understanding of one of the most complex and contested criminal cases in recent British history.

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