The mother of a newborn boy murdered by Lucy Letby has told an inquiry into the crimes of the killer nurse that she carries the “very, very painful” guilt that other babies may have survived if she had agreed to a postmortem examination of her son.

The woman, whose twin boys were attacked by the neonatal nurse, told the Thirlwall inquiry she was informed as she cradled her dead son that a postmortem was unnecessary because his cause of death was known.

She said she agreed with the doctor’s decision at the time but now felt it was “impossible” and “unfair” to have had to decide so soon after his death.

Letby, 34, was found guilty of murdering the newborn boy, known as Child E, by injecting air into his bloodstream and of attempting to poison with insulin his twin brother, Child F, at the Countess of Chester hospital in August 2015.

She is serving a whole life prison term after being convicted of murdering six other newborns and attempting to murder another six between June 2015 and July 2016, when she was removed from the hospital’s neonatal unit.

Child E’s mother told the inquiry into the killings that the events at the Countess of Chester hospital had “changed the course of our lives completely” and left them “grieving in so many different ways”.

She told the chairperson, Lady Justice Thirlwall, that a consultant who can only be identified as Dr ZA told her as they cradled her dead son that a postmortem was unnecessary because she believed he had died as a result of necrotising enterocolitis (NEC), an intestinal disease that affects some premature or very low birth weight infants.

A postmortem examination is not always required because the cause of death may be known or because it is likely that the death was due to natural causes.

Child E’s mother said she wished she had insisted on the postmortem because it may have ruled out NEC and found something suspicious. She said: “The criminal trial was very in depth and it took us through our children’s lives essentially hour by hour and to find out that Child E had had that significant bleed to the point of it being very, very unusual – but for no postmortem to be warranted from that made me question why.

“If it was so unusual, and so out of the blue, why on earth was a postmortem not given any weight to? If there was nothing on Child E’s X-ray to show any signs of NEC, why was a postmortem not mandatory? Why was it left for me to make that decision?

“Again I feel guilty for not requesting that because if that had been requested … and something would have been on it, there’s a lot of babies that could’ve not been involved in this case and it could’ve stopped there.

“And that weighs very, very heavily on me because that decision was ultimately ours and that’s very, very painful to think about. So I carry our grief but the sadness of other families because it should never have gone past that point.

“It was the same in the criminal trial when I realised that the insulin reading was there and it was seen and nothing was done. That could’ve been an end to this whole horrendously sad turn of events but it wasn’t.”

Letby has been convicted across two trials of going on to murder three babies and attempting to murder another four between September 2015 – the month after Child E’s death – and July 2016.

The inquiry heard that Dr ZA had apologised for the decision not to pursue a postmortem, which Child E’s mother said was the first apology she had received from anyone at the Countess of Chester hospital. She told the inquiry that the consultant wrote to her last September to apologise personally, “which was a really kind gesture from her”.

Concluding her evidence, Thirlwall thanked Child E’s mother and told her: “You have absolutely nothing to feel guilty about and you have nothing to blame yourself for – nothing at all. On the contrary, it is we who have to be grateful to you.”

Child E’s mother said the first time she was aware that there had been any unexpected deaths on the neonatal unit was in February 2017 when a taxi delivered a letter from the hospital advising her to read a report by the Royal College of Paediatrics and Child Health (RCPCH) due to be published online that day. This left her “absolutely mortified”, she said.