Senior doctors and scientists have told the BBC they have concerns about how crucial evidence was presented to the jury at Lucy Letby’s trials.

The BBC’s File on 4 has examined how expert witnesses helped to build the case against the former nurse.

The programme raises concerns about how courts grapple with cases of significant medical complexity – with the juries in Letby’s two trials presented with huge amounts of complicated medical evidence relating to each child.

The experts who spoke to the BBC raise questions about the amount of insulin she needed to harm babies in her care, the health condition of one of the babies she was convicted of murdering, and pathology findings presented to the jury.

A public inquiry is under way to establish how Letby was able to murder and injure babies. At its opening Lady Justice Thirlwall was scathing about those who have questioned the verdicts, saying this was causing “enormous additional distress to the parents”.

Last month some of the families of the babies gave evidence at the inquiry.

Each of the experts interviewed by File on 4 acknowledge how difficult it must be for the families to hear doubts raised about the trials. However, they say they feel so strongly about the evidence they felt compelled to speak out.

BBC File on 4 examines some of the most contentious statistical, scientific and medical evidence in the Lucy Letby trial. Listen to Lucy Letby: The Killer Questions

Available now on BBC Sounds and on BBC Radio 4 on Tuesday 1 October at 20:00, and Wednesday 2 October at 11:00.

More than 100 days of complex evidence was heard during Letby’s first trial, which ended in August 2023. She was found guilty of murdering seven babies and trying to kill six others between June 2015 and June 2016 at the Countess of Chester Hospital.

In a second trial held this year, a different jury found Letby guilty of attempted murder – after the first jury failed to reach a verdict. She is serving 15 whole-life sentences and four judges have dismissed her attempt to appeal these convictions.

Most of the experts File on 4 spoke to were not present at the trials, and they don’t offer an opinion on her guilt. They have studied key medical evidence presented in court. Their concerns – that some of it was misinterpreted – form part of the growing speculation around her convictions.

It comes after Letby’s new lawyer, Mark McDonald, told the BBC he plans to take her case to the Criminal Cases Review Commission (CCRC), which investigates alleged miscarriages of justice.

At her first trial Letby was found guilty of attempting to murder two babies – referred to in court as Baby F and Baby L – by adding insulin to intravenous feed bags.

The prosecution said both babies were doing well until Letby attacked them, and that it was suspicious she later searched for the parents on Facebook.

The prosecution alleged it would have taken only a few drops to poison each baby, but File on 4 has spoken to a team of mechanical and chemical engineers who disagree – and who are speaking about their calculations for the first time.

Prof Geoff Chase, from the University of Canterbury in New Zealand, has been modelling how insulin works in pre-term babies for more than 15 years. He worked with chemical engineer Helen Shannon on a mathematical model that calculated significantly higher quantities of insulin would be needed to harm babies F and L, and to generate insulin levels seen in their test results. In the case of Baby L they calculated it could be as much as 20-80 times more.

There was no evidence in the trial to suggest significant quantities of insulin had gone missing on the ward.

Speaking to File on 4, another expert expressed concerns about the use of the same blood test results at the trial – something others have questioned in the media.

Dr Adel Ismail – a world-leading expert in the test – told the BBC he believes the immunoassay blood test can produce misleading results.

“In my research, I found the error rate is one in 200,” he said, and added that a second, confirmatory test in such cases was “absolutely vital”. In the case of Baby F and Baby L follow-up tests were not carried out by the lab.

Some experts, however, say the tests are good enough to rely on one set of results. The hospital didn’t order further tests because both babies recovered soon after.

Letby was also found guilty of murdering a baby referred to in court as Baby C.

Key to the case was an X-ray taken on 12 June – it was referred to repeatedly during the first trial. In pre-trial reports two prosecution witnesses said it showed the baby had a swollen stomach “most likely due to deliberate” pumping of air into his feeding tube.

However, neonatologist Dr Michael Hall – who has spoken publicly about his concerns before, and has written to the chair of the public inquiry – told the BBC: “There are a number of possible explanations for there being excess gas there.”

Dr Hall, who was consulted by the defence but never called to give evidence, said it is likely to have been caused by the respiratory support the baby was receiving and said the X-ray suggested there was a bowel obstruction.

Letby was not working on the day the X-ray was taken and had not been on shift since before the baby was born – information the jury heard in her first trial. Letby’s former barrister Ben Myers also highlighted these details in his closing argument.

In his summing up the judge made clear to the jury this X-ray had been taken the day before Baby C collapsed, though he didn’t remind them Letby hadn’t been on shift. At appeal, the prosecution said Letby could have visited the hospital while off shift, but didn’t put forward any evidence that she was there.

The BBC has also spoken to five senior clinicians who reviewed Baby C’s medical notes made public at trial – although only one had access to the baby’s complete medical history. They all noted the baby was high risk and should have been in a higher level unit.

Prof Colin Morley, a retired professor of neonatology from the University of Cambridge, told File on 4 he was “very confident” Baby C died of natural causes.

At the trial, the X-ray was not the only evidence used to convict Letby on this charge. The prosecution argued text messages showed she was desperate to get into the room where Baby C was being treated, even though she wasn’t his designated nurse. Another nurse said she found Letby standing over the baby’s cot when he collapsed. After he died, Letby again searched for his parents on Facebook.

Baby O was one of triplet brothers born in good condition in June 2016. He was stable, Letby’s first trial was told, until the afternoon of 23 June when he suffered a “remarkable deterioration” and later died.

The pathologist for the prosecution, who reviewed the case, said he believed Baby O had suffered an “impact injury” to his liver akin to a road traffic collision.

There was other evidence used to convict Letby of Baby O’s murder. She objected to the baby being moved to another area of the unit to be more closely monitored. She was accused of falsifying medical notes, and there was a rash which prosecution experts said was consistent with air being injected into the baby’s veins.

However, a leading senior perinatal pathologist told File on 4 she agrees with the original post-mortem on Baby O – that his liver injury and death were by natural causes.

The pathologist – who asked not to be identified because of the controversial nature of Letby’s case – said she has seen this kind of liver damage at least three times in her career. Each time there were natural causes.

None of the experts who spoke to File on 4 made any evaluation of Letby’s guilt or otherwise, but added their concerns to growing speculation about how complex medical evidence was presented at her trials.

In August, 24 experts wrote to the government to share their concerns over the way statistics and the science around newborn babies was presented to the jury at the former nurse’s first trial.

The Crown Prosecution Service told the BBC: “Two juries and three appeal court judges have reviewed the evidence against Lucy Letby, and she has been convicted on 15 separate counts following two separate trials.”

It said in May the Court of Appeal dismissed Letby’s leave to appeal on all grounds – rejecting her argument that expert prosecution evidence was flawed.

This is a distressing case, so if you – or someone you know – need help after reading about it, the details of organisations offering assistance can be found on the BBC Action Line.