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Fresh controversy has emerged surrounding the conviction of former neonatal nurse Lucy Letby after claims that a potentially significant medical statement supporting her innocence was not disclosed to her defence team during her trial.

Letby, now 35, was convicted in 2023 of murdering seven babies and attempting to murder six others while working at the Countess of Chester Hospital. The case was one of the most shocking criminal trials in modern British history and led to a public inquiry examining how the deaths occurred and whether hospital management failed to act sooner.

However, new reporting has raised questions about whether key evidence may have been withheld from the defence.

A statement provided to police by paediatrician Dr Astha Soni reportedly disputed part of the prosecution’s medical case involving insulin poisoning. The document suggested that high insulin levels observed in one baby on the neonatal unit, referred to as Baby Y, may have been caused by a rare genetic condition that produces excess insulin rather than deliberate poisoning.

Baby Y was not one of the infants Letby was accused of harming. However, the prosecution relied on evidence about insulin poisoning in other cases to support allegations that Letby attempted to murder two babies by administering insulin.

According to reports, Dr Soni’s statement contradicted the conclusions of prosecution expert Dr Dewi Evans, a retired paediatrician who played a central role in the case against Letby. Evans had told investigators that the insulin detected in the baby’s blood must have come from an external source.

Critics argue that the existence of an alternative medical explanation could have challenged the prosecution’s broader claims about insulin poisoning on the ward.

The statement was reportedly never passed to Letby’s defence team by the Crown Prosecution Service. Legal experts say that if the document had the potential to undermine the prosecution’s case or assist the defence, it should have been disclosed under standard legal disclosure rules.

Lord Ken Macdonald KC, a former Director of Public Prosecutions, told the UnHerd website that the statement appeared to meet the legal threshold for disclosure before trial.

Further intrigue has surrounded the document after it briefly appeared on the website of the public inquiry examining the Letby case, led by Lady Justice Thirlwall. The statement reportedly appeared online on February 17 before later being removed.

The inquiry itself is currently examining the circumstances surrounding Letby’s crimes and how they went undetected for so long within the hospital system. However, some critics argue the inquiry assumes Letby’s guilt and should be paused while new questions about the evidence are examined.

In recent months, several medical experts have also raised doubts about aspects of the prosecution’s medical conclusions. Neonatologists Svilena Dimitrova and Neil Aiton have compiled reports challenging evidence presented at trial in relation to three of the baby deaths.

Separately, a panel of specialists reviewing elements of the case reportedly concluded that there was no clear medical evidence of deliberate harm in some of the cases and suggested the deaths could have resulted from natural causes or poor medical care.

Another claim reported by UnHerd states that juries in Letby’s two trials were not informed that a respiratory virus had been circulating in the neonatal unit shortly before one of the babies died.

The Crown Prosecution Service has declined to comment on the specific claims about the undisclosed statement but emphasised that Letby was convicted following two trials and multiple jury verdicts.

Letby is currently serving a whole-life prison sentence after being convicted of the murders and attempted murders.

The new allegations about undisclosed evidence and disputed medical findings are likely to intensify debate over the safety of the convictions as the public inquiry continues its work.