A senior coroner’s officer who first reviewed the deaths of babies at the Countess of Chester hospital for Cheshire police in 2017 now believes Lucy Letby has suffered a miscarriage of justice.

Stephanie Davies, who was given three hours to carry out her review, was told it was key to detectives deciding to commence an investigation into the former neonatal nurse.

In her first interview, with the Guardian and Channel 4 News, Davies said she had become increasingly alarmed since December, when she learned that the hospital doctors had not reported a key medical procedure on one of the babies to the coroner at the time. She has since found the explanations of new medical experts, who have publicly contested the prosecution arguments, compelling.

Last month, Davies wrote to Cheshire’s senior coroner explaining her original involvement. “I am now extremely concerned that the convictions of Ms Letby are wholly unsafe,” she wrote.

Letby, who was a nurse in the hospital’s neonatal unit, was convicted of murdering seven babies and attempting to murder seven more, and sentenced to a whole life in prison for each offence. The court of appeal twice last year refused her permission to appeal.

Letby’s lawyer, Mark McDonald, has applied to the Criminal Cases Review Commission (CCRC), arguing that the convictions are unsafe and should be referred back to the court of appeal.

A panel of international experts led by the world renowned neonatologist Dr Shoo Lee, who were instructed by McDonald, have publicly argued that Letby has been wrongly convicted, that there were no murders, and the babies died from natural causes and instances of poor care in the hospital. Lawyers representing the babies’ families have been adamant that the convictions are safe and rejected the new experts’ opinions.

Two specialist consultant neonatologists, Dr Neil Aiton and Dr Svilena Dimitrova, produced a report for McDonald on a triplet who died at the hospital, anonymised at Letby’s trial as Baby O.

Davies said she was taken aback at the account of their report given at a press conference in December, that in trying to relieve swelling in the abdomen, a doctor had mistakenly inserted a needle with a cannula into the liver, and that this was a cause of the baby’s death. Lee’s panel has also said the needle may have penetrated the baby’s liver.

That medical procedure was considered at the trial, where the doctor who inserted the needle, the neonatal unit’s clinical lead, Dr Stephen Brearey, said it was “nowhere near” the liver. Prosecution medical experts argued that the procedure was not significant to the harm the baby suffered. Letby was found guilty of inflicting the liver damage, and injecting air into the baby’s stomach and veins.

In December, Davies noted from her original review that the hospital doctors had not reported the procedure involving the needle to the coroner’s office. That meant it was not mentioned in the summary of circumstances surrounding the baby’s death, sent by the coroner’s office to the pathologist carrying out the post-mortem.

In his postmortem report, the pathologist did identify bleeding from a rupture around the liver as the cause of Baby O’s death, and said it had been caused by “prematurity”.

Davies said: “I don’t know why the hospital doctors didn’t include the detail regarding the needle.

“I believe it could have made a difference to the post-mortem if it was highlighted. An inquest may well have been held, there may have been a finding that the baby died due to a medical procedure, and there may never have been a police investigation.”

In her statement to the public inquiry chaired by Lady Justice Thirlwall, Davies explained that in early May 2017 she was asked to attend a “Gold Group” meeting of senior Cheshire police officers, when they were considering whether to investigate the concerns raised by hospital doctors about Letby. The assistant chief constable, Darren Martland, asked Davies to review the post-mortems and records for the babies who had died, and provide an opinion the next morning.

Senior coroner’s officers lead teams investigating and collating evidence about deaths on behalf of coroners. Davies, who had undertaken specialist investigative training, had been promoted to senior coroner’s officer in March 2017.

In her review, Davies reported there were gaps in the explanations of how some babies had died. This was because some causes of death were quite general, including prematurity given for the rupture around Baby O’s liver.

“I reported that there were missing jigsaw pieces, that the deaths of the babies hadn’t been fully explained,” Davies says. “I said if the police went ahead with an investigation, they may find those missing pieces. But I did not say, or see any evidence, that any of the babies had been deliberately harmed.”

Davies says her manager, then Det Insp Paul Hughes, who became the senior investigating officer, told her when passing her office one day that her review was one of the main reasons they decided to launch a criminal investigation.

The case of Baby O became key to hardening the investigation into Letby. Cheshire police appointed Dr Dewi Evans, a retired paediatrician, as their medical expert. Evans has said the police showed him Baby O’s medical notes at his first meeting, and within 10 minutes, he saw the liver damage and said it must have been inflicted deliberately.

Davies says she was surprised when Letby was convicted, but she assumed the trial must have established the truth. In her Thirlwall statement, she cautioned that it had been a “near miss”, because the police were not going to investigate before her review. However, during the last six months, she has become increasingly concerned. “The new experts’ medical explanations make sense to me,” she says. “They have filled the gaps I saw when I did my review – much more than the police and prosecution case against Lucy Letby.

“I stand by the review I did, because I reported that there were gaps. But I feel almost guilty that it contributed to a police investigation being started, which led to convictions that I now believe are unsafe and a miscarriage of justice.”

Davies lost her job at Cheshire police in 2023 after a different review she conducted, which challenged the outcomes of two murder investigations. She consulted a small number of experts to further her research, and her report was leaked to the Sunday Times – not by her – then widely reported in the media, including by the Guardian.

Cheshire police pursued Davies for gross misconduct, alleging that she breached duties of confidentiality when she sent details to the experts. She resigned from the force before her disciplinary hearing.

Responding to Davies’ concerns about the Lucy Letby case, a Cheshire police spokesperson described Davies’s former position as “an administrative role within the Cheshire coroner’s office” and said she was “neither formally medically, nor legally trained”.

“A disciplinary hearing in February 2023 found allegations of gross misconduct by Stephanie Davies unrelated to the Lucy Letby case to be proved and had she not already resigned, she would have been dismissed without notice,” the spokesperson said. “Cheshire constabulary strongly refute the credibility of these claims.”

Davies said the senior coroner’s officer role was investigative, and rejected any suggestion that she was not well qualified for the work she did.

Dewi Evans said he stood by the opinions he gave about Baby O. The hospital and Brearey declined to comment.

The Guardian approached the solicitor representing the parents of the triplets, Babies O and P. He said that they did not wish to comment.