The registrar, who worked alongside Letby at the Countess of Chester Hospital, was described by prosecutors during her trial as having been the focus of an intense personal attachment. Letby, however, denied that they had been romantically involved and maintained that he was simply her best friend.

The doctor, who was in his 50s, died last month after reportedly being ill for some time. Sources said he had subsequently moved to another hospital, and his funeral has now taken place.

Because of the sensitivity surrounding his role in the case, the medic was granted anonymity during Letby’s ten-month trial at Manchester Crown Court and was identified only as Doctor A.

His evidence was one of the more emotional moments of the proceedings.

Letby was said to have broken down and cried while he gave evidence against her, having previously maintained that their relationship was one of close friendship rather than romance.

The court heard that the pair had worked together at the Countess of Chester Hospital in 2016, two years before Letby was first arrested in connection with the deaths and collapses of babies on the neonatal unit.

Their friendship included messages, social outings and trips away from the hospital.

More than 1,300 Facebook messages between the pair over a three-month period in 2016 were examined during the trial.

In one exchange, Doctor A reportedly told Letby that she was “one of the few nurses I would trust with my own children,” highlighting the level of professional confidence he appeared to have placed in her at the time.

The court also heard evidence about their time together outside the hospital.

Letby and the doctor travelled to London together and went on walks and other days out, details that prosecutors later used as part of their argument about the nature of her feelings toward him.

Private messages and notes discovered during the investigation also became significant.

Letby referred to the doctor as “sweetie” in messages, while notes found at her home reportedly described him as “love.”

The prosecution argued that the registrar was more than simply a friend in Letby’s mind and that her feelings toward him formed part of the background to some of the incidents investigated by police.

Prosecutors claimed that Letby knew the doctor would respond when a baby became seriously ill and suggested that this knowledge could have influenced her actions.

According to the prosecution case, Letby’s attachment to the doctor was one of the possible motivations behind her actions because she knew that a deterioration in a baby’s condition could bring him rushing to the neonatal unit.

The doctor’s own account later painted a more complicated picture of the relationship.

During evidence to the public inquiry into Letby’s crimes in 2024, he said he believed he had been “misled” and possibly “manipulated” by her.

That testimony came after Letby’s conviction and as investigators and officials continued examining how the crimes were able to take place and whether warning signs had been missed.

Letby was convicted of murdering seven babies and attempting to murder seven others between June 2015 and June 2016.

She was subsequently sentenced to a whole-life order, meaning she is expected to spend the rest of her life in prison.

Letby has consistently maintained her innocence.

Her convictions have also become the subject of continuing controversy, with a group of doctors arguing that some of the babies’ deaths and collapses could have been caused by natural medical conditions or problems with the care they received.

The Criminal Cases Review Commission is currently considering evidence presented by that group.

The death of Doctor A therefore comes as the wider debate surrounding the case continues.

His involvement was particularly significant because he occupied a position of trust within the neonatal unit and was someone Letby appeared to regard as an important figure in her professional and personal life.

At the time they worked together, however, there was no public suggestion that their friendship would later become relevant to one of the UK’s most notorious criminal investigations.

The relationship only became part of the prosecution narrative after investigators examined Letby’s communications, movements and interactions with colleagues.

The messages between Letby and Doctor A provided prosecutors with a detailed record of their friendship.

The sheer volume of communication was striking, with more than 1,300 Facebook messages exchanged in just three months.

The contents helped prosecutors portray a relationship that extended well beyond ordinary workplace contact.

Yet Letby rejected the suggestion that she had been in a romantic relationship with the doctor.

Her position was that they were extremely close friends, despite the affectionate language contained in some of their communications.

The doctor himself was also placed in an extraordinarily difficult position when he was called to give evidence.

He had known Letby as a colleague and friend and had apparently trusted her professionally.

His earlier message telling her that she was among the nurses he would trust with his own children demonstrated the confidence he had once placed in her.

Years later, he was giving evidence that prosecutors believed could help establish the circumstances surrounding the deaths and attempted deaths of vulnerable babies.

His evidence also illustrated how dramatically the case changed the lives of people who had worked alongside Letby.

Colleagues who had once regarded her as a trusted member of staff were forced to reconsider their experiences as investigators reconstructed events inside the neonatal unit.

The doctor later told the public inquiry that he felt he had been misled and possibly manipulated, suggesting that his understanding of his relationship with Letby changed significantly after the allegations emerged.

His death means that one of the people who occupied a central position in Letby’s professional and personal world is no longer available to provide further testimony.

Sources said he had been ill before his death and that he had moved to another hospital after his time working alongside Letby.

His funeral has since been held.

The circumstances of his death have not been linked to the criminal investigation, and there is no suggestion in the information available that his death was connected to Letby or the case.

Instead, his death closes another chapter in a story that continues to generate questions about the relationships, decisions and missed opportunities surrounding the neonatal unit.

For Letby, the legal battle is far from over.

Although she is serving a whole-life order at Bronzefield Prison in Surrey, she continues to deny murdering the babies and attempting to kill others.

The Criminal Cases Review Commission’s consideration of the new medical evidence could potentially become significant for her future legal challenges.

The panel of doctors involved has argued that natural causes and poor care may provide alternative explanations for some of the deaths and collapses attributed to Letby.

Those claims remain disputed, and her convictions currently stand.

Meanwhile, the death of Doctor A has added a poignant development to the continuing aftermath of the case.

He was once described in court as a close friend and was portrayed by prosecutors as someone whose attention mattered deeply to Letby.

He later became a prosecution witness, telling the public inquiry that he believed he had been misled and possibly manipulated.

His story reflects the extraordinary transformation experienced by many people connected to the case: ordinary professional relationships were revisited through the lens of a devastating criminal investigation, and friendships that once appeared unremarkable became evidence examined in court.

As Letby’s legal challenges continue, the evidence surrounding her relationship with Doctor A will remain part of the extensive record of the case.

But for those who knew him outside the courtroom, his death marks the loss of a doctor whose life became unexpectedly intertwined with one of Britain’s most closely followed criminal cases.