Killer nurse Lucy Letby has been interviewed by detectives in prison over the suspicious deaths of more infants.

The 34-year-old was questioned under caution over unexpected deaths and collapses of babies at her former place of work, the Countess of Chester Hospital and, for the first time, at Liverpool Women’s Hospital, where she trained as a student.

She is already serving a rare whole-life sentence for murdering seven babies and trying to kill seven others while working as a neo-natal nurse at the Countess of Chester Hospital.

Cheshire Police said in a statement: “We can confirm that, following agreement, Lucy Letby has recently been interviewed in prison under caution in relation to the ongoing investigation into baby deaths and non-fatal collapses at the Countess of Chester Hospital and the Liverpool Women’s Hospital. Further updates will follow.”

Detectives are reviewing the care of some 4,000 babies admitted to hospital while 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 previously emphasised that only those cases highlighted as medically concerning would be investigated further.

Letby, from Hereford, is serving 15 whole-life orders after she was convicted at Manchester Crown Court of murdering seven infants and attempting to murder seven others, with two attempts on one of her victims, between June 2015 and June 2016.

She is currently in HMP Bronzefield in Ashford, Surrey, which caters for category A women prisoners.

The Thirlwall Inquiry, looking into the Letby case, is currently continuing in Liverpool.

Dr Stephen Brearey, the senior paediatrician on the neo-natal unit at the Countess of Chester, told the public inquiry last month that he believes she is “likely” to have murdered or attacked more babies before she killed her first victim, a premature twin boy known as Baby A, in June 2015.

Dr Brearey told the hearing: “On reflection I think it’s likely that Letby didn’t start becoming a killer in June 2015, or didn’t start harming babies in June 2015.

“I think it’s likely that her actions prior to then over a period of time changed what we perceived to be abnormal.”

Dr Brearey said the reporting culture on the unit was good and staff were very aware to report things they thought had gone wrong, but “in retrospect” he believed some of the incidents between June 2015 and June 2016, and “probably before”, were deteriorations of babies that could have triggered further investigation.

Peter Skelton KC, representing families of Letby’s victims, said: “Child A was murdered on June 8 2015. Is it your view that she (Letby) had murdered or assaulted children in your hospital prior to that date?

Dr Brearey said: “I think that’s likely, yes.”

The inquiry has heard that babies’ breathing tubes became dislodged on 40% of shifts Letby worked at Liverpool Women’s Hospital between October and December 2012 and January and February 2015.