Prosecutors have blamed the mysterious deaths of seven babies in a British neonatal unit on the “constant malevolent presence” of one young nurse accused of poisoning the infants.

Lucy Letby, 32, is standing trial for the deaths of five boys and two girls, and attempted murders of another 10 newborns at the Countess of Chester hospital.

Manchester Crown Court heard that a “poisoner was at work” in the hospital, injecting the babies with insulin or air or pumping dangerous levels of milk through a feeding tube.

Ms Letby, of Hereford, has denied the 22 charges, some of which relate to multiple attempts to murder some of the infants.

Lucy Letby appears in the dock at Manchester Crown Court. Image: Elizabeth Cook

Prosecutor Nick Johnson KC said the busy general hospital had noticed a spike in baby mortalities from January 2015, the BBC reports.

“Prior to January 2015, the statistics for the mortality of babies in the neo-natal unit at the Countess of Chester were comparable to other like units,” he said.

“However, over the next 18 months or so, there was a significant rise in the number of babies who were dying and in the number of serious catastrophic collapses.”

Hospital consultants noticed the alarming rise in babies deteriorating unexpectedly and medics could not account for the suspicious deaths.

“Having searched for a cause, which they were unable to find, the consultants noticed that the inexplicable collapses and deaths did have one common denominator,” Mr Johnson told the court.

“[It was] the presence of one of the neonatal nurses – and that nurse was Lucy Letby.”

A “painstaking review” uncovered a common thread in the deaths – the nurse on duty.

“Many of the events in this case occurred on the night shifts,” Mr Johnson said.

“When upon Lucy Letby was moved on to day shifts, the collapses and deaths moved to the day shifts.

“They were all the work of the woman in the dock, who, we say, was the constant malevolent presence when things took a turn for the worse for these 17 children.”

The Countess of Chester Hospitalhad a high infant mortality rate while Lucy Letby worked there. Photo: Getty

The BBC reports that Ms Letby tried to kill one child three times, while another died as a result of being injected with air.

“Sometimes a baby that she succeeded in killing she did not manage to kill the first time she tried, or even the second time, and in one case even the third time.”

In one of several cases detailed in court, Mr Johnson told of one half of two sets of twins who were poisoned within days of being born but survived thanks to medical staff.

The trial may last up to six months.