Detectives are continuing to review the care of some 4,000 babies admitted to hospital while Lucy Letby was working as a neonatal nurse.

The period covers her spell at the Countess of Chester Hospital from January 2012 to the end of June 2016, and includes two work placements at Liverpool Women’s Hospital in 2012 and 2015.

Cheshire Police emphasised that only those cases highlighted as medically concerning would be investigated further.

They added the review at Liverpool Women’s Hospital does not involve any deaths.

Senior investigating officer Detective Superintendent Paul Hughes said: “This does not mean we are investigating all 4,000. It just means that we are committed to a thorough review of every admission from a medical perspective, to ensure that nothing is missed throughout the entirety of her employment as a nurse.”

Detective Superintendent Paul Hughes, right, said police will examine every admission into a neonatal unit over the duration of Lucy Letby’s career (PA)

Mr Hughes told the PA news agency: “We would be foolish if we were to think we have gathered all cases that Lucy Letby could have touched in one go.

“We are proud of our investigation but we are not that good to say we have got everything in one go.

“So we are committed to doing an overarching investigation looking at every single baby’s admission into neonatal unit for the entire footprint that Lucy Letby has been employed.

“There are some cases which have been initially highlighted to us at the moment as concerning by an independent review, and those parents have been informed about where we are with that investigation and are being supported.”

Two experienced neonatologists are looking through the medical notes of children to identify in very broad terms any unexpected and unexplained collapses, he said.

He said if an issue is identified then it will be passed to the particular hospital to “overlay their knowledge” of the case before a decision is made on whether to take the matter any further.

Mr Hughes said: “We are not narrowing it down to Lucy Letby. So a doctor could say ‘I have a concern about a case here’ and she might not even have been there.

“It’s not specific to her because it would be too directive.”

Following the arrest of Letby in July 2018, police said their investigation was looking at 17 deaths and 15 non-fatal collapses between March 2015 and July 2016.

Mr Hughes said a number of those deaths were no longer under investigation because a natural, or biological, cause had been identified.

The hospital previously said that two babies died on the unit in 2013 and there were three deaths in 2014.