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.