A senior doctor has told the Lucy Letby public inquiry that she dismissed the thought of a poisoner on the loose at a neonatal unit as “so fantastical” it could not be true.

Child serial killer nurse Letby attempted to murder one of her 14 victims, Child F, in early August 2015 by giving him insulin shortly after she murdered his twin brother, Child E, and following her murders of three other infants in June 2015.

Letby’s attack spree at the Countess of Chester Hospital’s neonatal unit continued until June 2016 with the murders of two of a set of triplets.

Giving evidence on Monday, a consultant paediatrician, who cannot be identified for legal reasons, told the Thirlwall Inquiry into events surrounding Letby’s crimes that she “deeply regrets” rejecting the idea that someone had deliberately harmed Child F after blood tests results revealed abnormally high levels of insulin.

The doctor, referred to as Dr ZA, recalled discussing the results when they came back from the laboratory about a week later with a colleague, by which time the youngster had recovered from the hypoglycaemic episode.

Dr ZA told the hearing: “We checked it was not prescribed.

“We checked that no-one else on the unit was on insulin thinking it was done accidentally but the idea that someone could be doing it deliberately was so fantastical and unlikely that it could not possibly have happened.

“At the time I dismissed the idea of someone deliberately administering insulin because it seemed so impossible but I deeply regret that is how I interpreted things, both for Child E and F’s parents and the parents of babies that happened subsequently.”

The doctor was asked by counsel to the inquiry Nicholas de la Poer KC if the case of fellow child serial killer nurse Beverley Allitt had not potentially crossed her mind as a “real life explanation”.

She replied: “No, I don’t know why not.

“I had sort of grown up with the knowledge of Beverley Allitt and what she did, and also later Harold Shipman, but it just never occurred to me that this was something that happened on my ward with the patients I was looking after.”

Dr ZA agreed she should have informed Child F’s parents about the hypoglycaemia and the abnormal blood results.

She also told the inquiry it was the “wrong decision” not to take any action over the results.

Richard Baker KC, representing families of Letby’s victims, suggested to Dr ZA she did not need to think it was an attempted murder to take action and it was a “serious safety issue” however the insulin got there.

The medic agreed: “Yes, I should have flagged it.”

By March 2016, the inquiry heard, she said she considered it possible that Letby may be harming babies, although she did not link the concerns to Child F’s blood test results until more than a year later when police had already been called in by the hospital.

Dr ZA said: “It was a general feeling of unease at the sudden, unexpected and unexplained nature of the events and the correlation with Letby being present but nobody sort of knew exactly what she was doing.

“It was that sort of uncomfortable association at that point and the lack of another plausible explanation.”

Following the deaths of triplets Child O and P in June 2016, she felt the association “had gone far beyond coincidence and her working pattern” and that Letby may be involved in some way “either by unconscious incompetence or by a deliberate act”.

Those concerns were made clear by the consultants to the hospital’s senior management, she said.

I very much took it to mean that if we continued to carry on raising our concerns then my job would be at risk

Letby was then moved from the neonatal unit to non-patient duties as the hospital commissioned an independent external rate into the increased mortality rate.

Dr ZA said: “It seemed initially they didn’t want to listen to our concerns and then over time as we became more persistent it seemed that they wanted us to be quiet and shut up about it.”

She recalled “very clearly” a meeting on January 26 2017 between the consultants and the executive directors group.

She told the inquiry: “There was a very adversarial atmosphere from the moment we first entered the room.

“The senior management, all of them, seemed to be speaking with the same tone and it very much seemed like we had been called into the headmaster’s office like naughty schoolchildren.

“Tony Chambers (the chief executive) said he was ‘drawing a line and we were not to cross it’ and it was said in quite a threatening tone.

“I very much took it to mean that if we continued to carry on raising our concerns then my job would be at risk.”

She went on: “When raising concerns we were initially ignored then later actively bullied and victimised.

“I think there was a false narrative developing that (paediatric services lead) Dr (Ravi) Jayaram and (neonatal lead) Dr (Stephen) Brearey were ringleaders rather than we were all professionals who had come to the same conclusion.

“Also I had some soul searching about at what point I would be happy to stop raising concerns and stop pushing.

“I decided that the only point I would feel that I could sleep at night was that if Lucy Letby was not working as a nurse or in a similar position…and even if I lost my job we should persist with this until we reached this outcome.”

Cheshire Police were not brought in to investigate the increased mortality rate until May 2017 and Letby remained at the hospital until her first arrest in July 2018.

Letby, 34, from Hereford, is serving 15 whole-life orders after she was convicted at Manchester Crown Court of murdering seven infants and attempting to murder seven others, with two attempts on one of her victims, between June 2015 and June 2016.

The inquiry is expected to sit at Liverpool Town Hall until early next year, with findings published by late autumn 2025.