A hospital nurse accused of murdering seven babies and attempting to murder 10 others was a “constant malevolent presence” in their care, a jury has heard.

Lucy Letby, 32, is alleged to have gone on a year-long killing spree while working at the Countess of Chester Hospital.

Letby, of Arran Avenue, Hereford, denies murdering five boys and two girls and attempting to murder another five boys and five girls between June 2015 and June 2016.

Opening the prosecution case on Monday at Manchester Crown Court, Nick Johnson KC said the Countess of Chester was a “busy general hospital” which included a neonatal unit that cared for premature and sick babies.

He said: “It is a hospital like so many others in the UK, but unlike many other hospitals in the UK and unlike many other neonatal units in the UK, within the neonatal unit at the Countess of Chester Hospital a poisoner was at work.

“Prior to January 2015 the statistics for the mortality of babies in the neonatal unit at the Countess of Chester were comparable to other like units.

“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.

“And these rises were noticed by the consultants working at the Countess of Chester and they searched for a cause.”

Mr Johnson went on: “Their concern was that babies who were dying had deteriorated unexpectedly. Not only that when babies seriously collapsed they did not respond to appropriate and timely resuscitation.

“Some of the babies who did not die collapsed dramatically but then – equally dramatically – recovered.

“Their collapse and recovery defied the normal experience of treating doctors.”

He continued: “Babies who had not been unstable at all suddenly deteriorated. Sometimes babies who had been sick but then been on the mend suddenly deteriorated for no apparent reason.

“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. The presence of one of the neonatal nurses and that nurse was Lucy Letby.”

He told jurors at Manchester Crown Court that at the relevant time the number of nursing staff at the Countess of Chester’s neonatal unit was between 25 and 30 nurses, along with about 15 nursery nurses.

Mr Johnson continued: “Many of the events in this case occurred on the night shifts.

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

Mr Johnson said as medics could not account for the collapses and deaths, police were called in and conducted a “painstaking review”.

He said: “That review suggests that in the period between mid-2015 and the middle of 2016, somebody in the neonatal unit poisoned two children with insulin.

“The prosecution say that the only reasonable conclusion to be drawn from the evidence you will hear is that somebody poisoned these babies deliberately with insulin. This was no accident.

“If we are right about that, the fact that there were two deliberate poisonings will help you when you are assessing whether the collapses and deaths of other children on the neonatal unit were because someone was sabotaging them or whether these were just a tragic coincidence.”

The collapses and deaths of all the 17 children concerned were not “naturally occurring tragedies”, Mr Johnson said.

He said: “They were all the work, we say, 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.”

Mr Johnson said the two children allegedly poisoned with insulin, who cannot be identified, were two baby boys, both born twins; the first born in summer 2015 and the other born in spring 2016.

Both were allegedly poisoned a couple of days after they were born.

“Lucy Letby was on duty when both were poisoned and we allege she was the poisoner,” Mr Johnson said.

“There’s a very restricted number of people who could have been the poisoner, because entry to a neonatal unit is closely restricted.”

Letby, wearing a blue jacket over a black shirt, earlier pleaded not guilty to seven counts of murder and 15 counts of attempted murder.

Family members of some of her alleged child victims sat in the public gallery listening as the names of the children were read out during her not guilty pleas.

On the other side of the public gallery sat the defendant’s parents, John, 76, and Susan, 62.

A court order prohibits reporting of the identities of surviving and deceased children allegedly attacked by Letby, and prohibits identifying parents or witnesses connected with the children.