New evidence seen by the BBC suggests more babies in Lucy Letby’s care were harmed – and in one case poisoned with insulin.

The former nurse was found guilty of murdering seven babies and attempting to murder seven others – including trying to kill two with insulin at the Countess of Chester Hospital neonatal unit between June 2015 and June 2016.

BBC One’s Panorama has seen documents which suggest a third baby may have also been poisoned within hours of Letby taking over the boy’s care.

Medical records reveal the infant’s blood sugar level plummeted and lab results indicated he had suspiciously high levels of insulin.

Panorama has also discovered that potentially life-threatening incidents involving infants occurred on almost a third of Letby’s 33 shifts while she was training at Liverpool Women’s Hospital in 2012 and 2015.

The programme’s revelations follow months of criticism of the prosecution’s case in her first trial. A number of experts have challenged the medical evidence used to convict Letby, as well as the way statistics were put forward in court.

In August 2023, the 33-year-old was sentenced to life in prison, with no chance of parole. Letby was then found guilty of attempting to murder a seventh baby at a second trial in July this year, and sentenced to a 15th whole-life prison term.

The nurse has been refused leave to appeal against the convictions from her first trial.

Panorama has examined mounting questions from leading statisticians and medical experts about the safety of her convictions.

But as part of the programme, new evidence has also emerged of other sick and premature babies potentially being harmed while in Letby’s care.

When the body produces insulin naturally, it also produces a substance called C-peptide. Typically, the level of C-peptide will be five to 10 times higher than the level of natural insulin.

Letby’s first trial heard blood tests from the two babies showed they had high levels of insulin and very low levels of C-peptide.

The prosecution argued the insulin must have been given to them rather than naturally produced.

Letby’s lawyers did not accept the insulin evidence used in court, but neither did they argue it was wrong. When she was cross-examined Letby herself accepted the two babies must have been poisoned, but denied it was done by her.

The new evidence seen by Panorama shows a blood test from a third baby being cared for by Letby in November 2015 also recorded very high levels of insulin and low levels of C-peptide.

Lab results indicated the insulin level was in excess of 6,945 picomoles per litre – a very high reading. Had the insulin been natural, the C-peptide level would have been between 35,000 and 70,000, but the blood test showed it was just 220.

At the time, consultants on the neonatal unit assumed the insulin must be natural. Tests later revealed the baby had congenital hyperinsulinism (CHI) – a condition where the body naturally produces too much insulin.

But four experts have told Panorama that CHI could not explain such an exceptionally high insulin reading for the infant – partly because of the low C-peptide level, but also because a baby with CHI would never produce that much insulin.

Medical records seen by Panorama show how quickly the boy became poorly after Letby came on duty. A blood test taken at 06:56 showed the infant had a normal blood sugar level of three millimoles per litre (mmol/L).

Letby started her shift at 08:00, and by 13:54 his blood sugar level had plummeted to one mmol/L – a dangerously low level, and a strong indication the baby had too much insulin.

The boy’s blood sugar level remained low throughout the nurse’s shift and he only recovered after she went off duty at 20:00.

Letby’s new lawyer, Mark McDonald, noted that the baby had a particular problem with regulating his own insulin. He also disputed the claim that the baby’s condition could not explain the extremely high insulin levels recorded by the lab.

“It may be said by one expert, but I have other experts that have a completely contradictory view,” Mr McDonald said. “I’m working night and day on this case. If I thought for a moment that [Lucy Letby] was guilty, I wouldn’t be doing this.”

Some experts have also questioned the accuracy of the simple test used to measure insulin in all three of the cases, known as the immunoassay method.

They point out there is another, more precise test and that only the more advanced test can definitively determine levels of insulin in the blood.

There are circumstances in which the immunoassay method can give flawed or misleading results, but the test is widely used and is usually accurate.

Panorama has spoken to leading experts on all sides of this debate. The programme found circumstances in which interference might occur are very unlikely in the context of the babies in the Letby case. It is even more unlikely that three lab tests conducted within months of each other would all be wrong.

It’s a point that Letby’s lawyer, Mr McDonald, disputes: “It is accepted by all sides that there’s an error rating [with the test], but it’s the percentage of the error rating that’s not accepted.”

Panorama has also discovered that potentially life-threatening incidents occurred on almost a third of Letby’s 33 shifts while training at Liverpool Women’s Hospital in 2012 and 2015.

In one case, from November 2012, a baby boy collapsed and water was subsequently discovered in his breathing tube – a highly irregular occurrence. The clinical notes confirm that the nurse looking after him was Letby.

In addition, a retrospective analysis showed that babies’ breathing tubes became dislodged on 40% of Letby’s shifts. The norm per nurse per baby was 1%.

Cheshire Police is continuing to investigate other cases the force believes the nurse may have been involved in, including the Liverpool incidents.

Mr McDonald plans to take Letby’s case to the Criminal Cases Review Commission (CCRC) to apply for it to be sent back to the Court of Appeal.

Reporter Judith Moritz, who has covered the case from the start, investigates the questions that have been raised about Lucy Letby’s conviction.

Watch Lucy Letby: Unanswered Questions on BBC iPlayer, or on BBC One on Monday 21 October at 20:00 (20:30 in Wales and Northern Ireland).