Cheshire police have launched an investigation into an unusually high rate of baby deaths and collapses at the Countess of Chester hospital over a 12-month period.

The hospital said it had contacted the police this month to help them rule out unnatural causes of death in its neonatal unit between June 2015 and June 2016. The force said it would examine the deaths of 15 babies and the collapses of six.

“Cheshire constabulary has launched an investigation, which will focus on the deaths of eight babies that occurred between that period where medical practitioners have expressed concern,” DCS Nigel Wenham said.

“In addition, the investigation will also conduct a review of a further seven baby deaths and six non-fatal collapses during the same period.”

Last November a report by the Royal College of Paediatrics and Child Health (RCPCH) found that staffing at the hospital’s neonatal unit, which reported a “higher than usual” number of baby deaths, was inadequate. Two babies died in the unit in 2013 and three died in 2014. In comparison, there were eight deaths in 2015 and five in 2016.

The review found no definitive explanation for an increase in mortality rates, but identified significant gaps in medical and nursing rotas, poor decision-making and insufficient senior cover. Last July the Countess of Chester hospital stopped providing care for babies born earlier than 32 weeks.

In a statement, the trust said it was confident its neonatal unit was safe to continue in its current form. “We will be working with staff to ensure that expectant mums and those individuals currently on the unit are provided with the necessary assurances to allow us to continue to care for them here at the Countess,” it read.

Wenham said that the investigation was in its very early stages, so police would not be able to provide any further details. “We recognise that this investigation will have a significant impact on all of the families involved, staff and patients at the hospital and the public,” he said.

“Parents of the babies are being updated on the investigation and will be supported throughout the process by specially trained officers. We are committed to carrying out the investigation as quickly as possible.”

The trust said it had sought the input of Cheshire police after the findings of the report from the RCPCH. The review made 24 recommendations for improvement, which the hospital said it had now undertaken.

A statement said the trust had “continuing concerns about the unexplained deaths” and were “very keen to understand that everything possible has been done to help determine the causes”.

Ian Harvey, the hospital’s medical director, said: “We are deeply sorry for the further distress and heartache this will cause. Throughout this we have never lost sight of the families left bereaved by the loss of their baby, and they will continue to be our main concern.

“At every point where the hospital has been able to share information with families and the public, we have done so. Approaching the police is not something we have undertaken lightly. This is to ensure we have been completely thorough in understanding what has happened here and to get the answers we and the families so desperately want.”

A solicitor for one of the affected families told the BBC: “Clearly the death of any child is a tragedy, but this is exacerbated in circumstances where questions remain unanswered.

“However, we are reassured that all steps are now being taken to investigate these deaths in their entirety, and we are hopeful that the investigation can provide some answers for the families of these children.”