Bold claim: a key expert witness in Lucy Letby’s case was under a fitness-to-practise investigation at the time he testified, yet the jury never knew. And this is where the story gets even more controversial...
A physician who provided crucial expert testimony on insulin poisoning for the prosecution in the Lucy Letby case was being investigated by the medical regulator for serious concerns about his fitness to practise when he gave evidence. The General Medical Council (GMC) opened its inquiry into Professor Peter Hindmarsh on the trial’s opening day in late 2022, amid allegations that he had harmed patients. The investigation continued when Hindmarsh returned to give evidence a second time three months later. Great Ormond Street Hospital had already referred Hindmarsh to the GMC after a formal probe led by his main employer, the University College London Hospitals NHS Foundation Trust (UCLH).
Crucially, the jury hearing Letby’s case—that led to a conviction for murdering babies and attempting to murder others at the Countess of Chester Hospital’s neonatal unit—was not informed about Hindmarsh’s regulatory probe. During the period of the GMC inquiry, and while Hindmarsh was still giving evidence, a medical tribunal had already imposed stringent restrictions on his professional work, stating he “may pose a real risk” to the public. The tribunal also flagged concerns that his ability to act as an expert witness could be compromised. Despite this, the tribunal allowed Hindmarsh to continue testifying for the prosecution. The Crown Prosecution Service argued to the defence that revealing the GMC investigation to the jury would be inappropriate since the allegations had not reached a final ruling.
Ultimately, the GMC investigation never reached a formal conclusion because Hindmarsh voluntarily erased himself from the GMC register, effectively closing the case without a regulatory finding. Details of the allegations against Hindmarsh at the time of his testimony were later uncovered by The Guardian.
Letby was found guilty of seven counts of murder and seven counts of attempted murder, with two trials spanning 2023 and 2024. She received 15 whole-life terms. Appeals were denied by the Court of Appeal. Letby has consistently maintained her innocence, and many supporters believe she was scapegoated for broader medical failures in the unit.
Since the convictions, prominent UK and international experts have challenged the prosecution’s medical case, including Hindmarsh’s evidence. Dr Shoo Lee, a well-known Canadian neonatologist who contends that the prosecution misrepresented his research, led a 14-member expert panel that, in findings published last year, concluded there was no medical evidence supporting Letby’s conviction. The panel unanimously determined that the babies died or deteriorated due to natural causes and substandard care, with no evidence of murder or insulin poisoning.
In February of the previous year, Letby’s legal team submitted an application to the Criminal Cases Review Commission (CCRC) to have the case referred back to the Court of Appeal. The CCRC is reviewing the application, aided by the panel’s reports and other expert input from 27 leading specialists.
‘Widespread and serious concerns’
Hindmarsh testified twice as a core prosecution witness during Letby’s first trial, which ran from October 2022 to August 2023 and is regarded as one of Britain’s lengthiest murder trials. A professor and consultant paediatric endocrinologist, Hindmarsh brought extensive experience in pediatric care to the proceedings, having worked at UCLH for decades and served as an honorary consultant at Great Ormond Street.
His first appearance was on 25 November 2022, where he supported the prosecution’s theory that two babies, labeled F and L, had been capable of insulin poisoning via contaminated infusion bags. However, the jury did not learn that Hindmarsh had been facing professional allegations in the months leading up to the trial. He remained at UCLH but had previously ended his contract with Great Ormond Street in July 2022.
Requests for clarity about Hindmarsh’s employment status during testimony were not fully resolved in court. The Guardian received a reply that Hindmarsh had been an honorary consultant at Great Ormond Street at that stage, though the record did not make clear the contract termination details. The jury was not informed of the broader misconduct allegations. In the months preceding his testimony, UCLH had led a formal investigation into Hindmarsh, and on 15 November 2022 Great Ormond Street referred “multiple and wide-ranging concerns” about Hindmarsh’s practice to the GMC. The regulator opened its own formal investigation on 25 November, the same day Hindmarsh first took the stand.
Subsequent developments included more inquiries: on 30 November Hindmarsh learned of the GMC probe, and on 21 December 2022 UCLH reported similar concerns to the GMC, noting Hindmarsh had been suspended pending a return to work under close supervision. Leaked documents cited by The Guardian describe concerns such as “harm to patients,” questions about his diagnostic and treatment decisions, unusual use of treatments in certain age groups, questionable investigations, and inadequate documentation.
By January 2023, a tribunal renewed restrictions on Hindmarsh, limiting him to work at UCLH with supervision, given the risk to the public and the potential impact on his credibility as an expert witness. Despite these concerns, Hindmarsh sought permission to continue serving as an expert witness, emphasizing that this work was unrelated to the regulatory referral. The tribunal permitted continued expert testimony, provided Hindmarsh informed any instructing party of his GMC investigation status.
This created a tension: the prosecution did not object to Hindmarsh’s role as an expert witness, but the question remained whether it was appropriate for him to testify while under investigation. The Crown Prosecution Service asserted there was no evidence the jury had been misled about Hindmarsh’s qualifications or experience.
Hindmarsh testified again on 24 February 2023, with the jury unaware of the GMC inquiry, the UCLH-led investigation, or the active restrictions from the tribunal. His testimony was pivotal for the prosecution, who argued those insulin-related cases were decisive, showing deliberate harm that could also inform the broader case against Letby. The judge in summation suggested that proof of deliberate harm in one case could justify inferences about others, even if the exact mechanism remained uncertain.
The insulin-related arguments stood out because they relied on medical blood tests to demonstrate poisoning, rather than more typical forms of evidence. Hindmarsh claimed the tests and medical records for babies F and L indicated hypoglycemia caused by insulin, supporting the prosecution’s theory that Letby injected insulin into the fluids in the bags. No direct forensic link to Letby was established.
Today, many experts who question Letby’s convictions have criticized Hindmarsh’s insulin testimony and the broader prosecution narrative. Some analyses argue that certain insulin tests can produce erroneous results and should not have formed the core of the case. They also question Hindmarsh’s calculations and suggest alternative explanations for the babies’ hypoglycemia, including possible failures in medical care.
The Guardian asked Hindmarsh for detailed remarks about the GMC and UCLH investigations, as well as whether he fully disclosed the investigations before testifying; his response was to offer no comment. A GMC spokesperson confirmed that Hindmarsh is no longer registered, having relinquished his registration on 14 November 2024.
