A hospital neonatal nurse accused of killing seven babies and trying to kill 10 others poisoned two infants deliberately with insulin, a British prosecutor said Monday.

Lucy Letby, 32, has been charged with murder in the deaths of five baby boys and two girls, and the attempted murder of five boys and five girls, while she worked at the Countess of Chester Hospital in northwest England between 2015 and 2016. Letby earlier pleaded not guilty to the charges.

Opening her trial at Manchester Crown Court, prosecutor Nick Johnson said that from 2015, the hospital saw a significant rise in the number of babies who were dying or suffering "serious catastrophic collapses".

Lucy Letby appearing in the dock at Manchester Crown Court, in Manchester, England. Illustration / Elizabeth Cook, AP

"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," he told the jury.

He said that when doctors couldn't find a cause, police were called in and conducted a review that suggested someone in the neonatal unit poisoned two infant boys with insulin two days after they were born.

Johnson said Letby was on duty when both were poisoned. He added that prosecutors believed the collapses and deaths of all 17 babies were the work of Letby, who he described as a "constant malevolent presence" at the hospital's neonatal unit when the children collapsed or died.

Family members of some of Letby's alleged victims sat in the courtroom, while Letby's parents were also present.

The trial is expected to last weeks, if not months.

Police launched an investigation into the deaths of a number of babies at the hospital in May 2017. Letby was arrested three times in connection with the deaths before she was charged in November 2020.