Trial process

Crown insulin expert's GMC investigation — undisclosed to jury

The prosecution’s claim: Prof. Peter Hindmarsh, Professor of Paediatric Endocrinology at University College London and Consultant Paediatric Endocrinologist at Great Ormond Street Hospital, was presented to the jury as a senior, eminent expert qualified to interpret the Roche Cobas insulin immunoassay results for Babies F and L. His interpretation of the insulin-to-C-peptide ratios as diagnostic of exogenous insulin administration was the principal scientific support for both insulin counts.

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Last updated 2026-07-14

3 min read

Prosecution claim

Prof. Peter Hindmarsh, Professor of Paediatric Endocrinology at University College London and Consultant Paediatric Endocrinologist at Great Ormond Street Hospital, was presented to the jury as a senior, eminent expert qualified to interpret the Roche Cobas insulin immunoassay results for Babies F and L. His interpretation of the insulin-to-C-peptide ratios as diagnostic of exogenous insulin administration was the principal scientific support for both insulin counts.

Counter-evidence

On the same day Prof. Hindmarsh began giving evidence at the 2022–2023 Letby trial, the General Medical Council opened a fitness-to-practise investigation into him. A subsequent Medical Practitioners Tribunal Service interim order imposed severe restrictions on his clinical work, stating that he 'may pose a real risk' to members of the public and that the allegations 'may have the potential to impact on his ability to act as an expert witness'. The jury was never told. On 14 November 2024 — after the original trial concluded and after the 2024 Court of Appeal refusal — Hindmarsh removed himself from the GMC register through voluntary erasure, which ended the GMC investigation without any regulatory finding. The post-conviction expert critique argues this non-disclosure to the jury is material because, under Criminal Procedure Rules Part 19, an expert witness owes a primary duty to the court and is obliged to disclose anything that bears on competence, credibility or independence. It is now one of the central grounds in the CCRC submission record on the insulin counts.

"The Medical Practitioners Tribunal Service concluded that the allegations against Professor Hindmarsh may have the potential to impact on his ability to act as an expert witness. The jury was never told." — Patient Safety Learning hub summary, 2025

What the jury heard

Prof. Peter Hindmarsh's expert evidence on the insulin assay results was presented uncontested by any disclosure of the parallel GMC investigation. The Crown's closing relied on his interpretation as the principal scientific underpinning of the two insulin counts.

What the Panel says

The Shoo Lee International Expert Panel's medical findings on the insulin counts were reached independently of the GMC issue. The Joint Expert Witness Insulin Report (May 2025) addresses the assay-methodology grounds; the GMC non-disclosure is a separate procedural ground engaging Criminal Procedure Rules Part 19.

What independent experts add

The GMC investigation opened the same day Hindmarsh began giving Letby trial evidence in late 2022.
A Medical Practitioners Tribunal Service interim order imposed restrictions on his work, citing 'real risk' to members of the public.
The Crown Prosecution Service told the defence it would oppose any attempt to disclose the GMC investigation to the jury, on the basis that the allegations had not reached a final adjudication. This active prosecution decision not to allow the jury to weigh the witness's parallel professional-conduct investigation is a procedural fact distinct from the underlying non-disclosure itself. Source: Guardian reporting by Felicity Lawrence and David Conn, via The Justice Gap secondary summary.
Voluntary erasure on 14 November 2024 terminated the GMC investigation without any regulatory finding being made.
Under Criminal Procedure Rules Part 19, expert witnesses owe a primary duty to the court and must disclose matters affecting competence, credibility or independence.
Dr Adel Ismail described the immunoassay results in the Letby case as 'completely worthless' in a 2025 BBC Panorama documentary, on the separate methodological ground.
The Royal Liverpool laboratory subsequently exaggerated insulin levels in a quality-control sample by almost 800% in a routine assessment a few weeks after testing Baby L's blood.