A doctor who gave expert evidence in the trial of Lucy Letby was being probed at the time by the medical regulator over claims he harmed patients.
The General Medical Council (GMC) opened an investigation into whether Professor Peter Hindmarsh was fit to practise on the first day he gave evidence in November 2022.
This was still going on when he gave evidence for a second time three months later, in what became one of the longest murder trials in British history.
The hormones specialist was an expert witness for the prosecution’s case that Letby attempted to murder two babies, referred to as F and L.
Professor Hindmarsh’s evidence supported the case that both had been poisoned with insulin injected into their fluid feed bags.
Great Ormond Street hospital, where he was an honorary consultant, reported him to the GMC after an investigation by his main employer, University College London hospitals trust (UCLH).
The jury, which convicted Letby of seven counts of murder and seven of attempted murder, was never informed about the GMC investigation or his contract being terminated by Great Ormond Street in July 2022.
Letby, who was a nurse on the neonatal unit at the Countess of Chester hospital, has always maintained she is innocent, and many leading experts have argued that the prosecution’s medical case, including Professor Hindmarsh’s evidence, was wrong.
Professor Hindmarsh’s evidence supported the case that two babies had been poisoned with insulin injected into their fluid feed bags
She was sentenced to 15 whole-life terms in prison, and Court of Appeal judges have turned down her applications to appeal.
While the GMC conducted its probe during the trial, a medical tribunal ordered restrictions on Professor Hindmarsh’s work.
It said: ‘There is information to suggest he may pose a real risk to members of the public, if he were permitted to return to unrestricted clinical practice, given the number and nature of the concerns, involving paediatric [child] patients.’
Leaked documents reveal the claims against Hindmarsh included ‘cases where harm to patients has been alleged’, according to The Guardian.
They also reportedly show concerns about his ‘diagnosis and treatment of patients’, ‘use of treatments in unusual age groups, without adequate monitoring’ and ‘inappropriate’ investigations and documentation.
The tribunal considered that the allegations against Professor Hindmarsh, a paediatric endocrinologist, ‘may have the potential to impact on his ability to act as an expert witness’. But it allowed him to continue giving expert evidence.
The Daily Mail understands the prosecution was not informed of the investigation until December 2022, after he had first given evidence.
The prosecution told the defence before his second appearance, but said it would oppose any attempt to inform the jury, on the basis that the GMC investigation had not reached a final adjudication.
In fact, it was never concluded, because he removed himself from the medical register. This ended the investigation and resulted in no regulatory findings against him.
Professor Hindmarsh told the court that blood tests and medical records of babies F and L showed they suffered from hypoglycaemia – low blood sugar – because they had been poisoned with insulin, most likely in their intravenous fluid bags.
The prosecution alleged that Letby had injected insulin into the bags, although there was no direct or forensic evidence that anyone had done so.
It said the cases of babies F and L were ‘incontrovertible’ evidence that someone was deliberately endangering infants.
And Judge Mr Justice Goss told the jury that if they were sure about deliberate harm in any one case, they could infer the same in other babies.
Experts have since said that the tests used to measure insulin are known to produce unreliable results.
The Criminal Cases Review Commission, the body that examines possible miscarriages of justice, is looking at Letby’s application to have her case sent back to the Court of Appeal.
In it, around 27 experts claim that each death and collapse she has been convicted of could be explained by natural causes or failures by the medical team.
A separate police investigation is underway into corporate manslaughter and gross negligence manslaughter at the Countess of Chester Hospital. Three former senior leaders were arrested in June 2025 and remain on bail pending further inquiries.
Peter Hindmarsh declined to comment. The GMC did not provide a statement, but noted: ‘Where a doctor has an interim suspension or conditions in place during an investigation, this is published clearly on our public-facing medical register.’
