A female healthcare worker has been arrested on suspicion of murdering eight babies and attempted murder of another six babies, according to Cheshire Police.

The woman, who has not been named, was identified after an investigation of the neonatal unit at the Countess of Chester Hospital.

Police launched the probe in 2017 following a “greater number of baby deaths and collapses than normally expected” between June 2015 and June 2016.

The woman’s arrest on Tuesday morning represented a “significant step forward” in the investigation, according to lead investigator Detective Inspector Paul Hughes. But he cautioned enquiries were still “very much active and ongoing” at this stage.

The investigation originally centred on the deaths of 15 babies and six non-fatal collapses, according to police. But its scope has now widened to cover the deaths of 17 babies and 15 non-fatal collapses between March 2015 and July 2016.

Police have not confirmed whether the woman arrested was a doctor, nurse or other healthcare professional.

“This is a highly complex and very sensitive investigation and, as you can appreciate, we need to ensure we do everything we possibly can to try to establish in detail what has led to these baby deaths and collapses,” said DI Hughes.

“As a result of our ongoing enquiries we have today (Tuesday 3 July) arrested a healthcare professional in connection with the investigation. She was arrested earlier this morning on suspicion of murder in relation to eight of the babies and attempted murder in relation to six of the babies and is currently in custody.

“Whilst this is a significant step forward in our enquiries it is important to remember that the investigation is very much active and ongoing at this stage. There are no set timescales for this coming to a conclusion but we remain committed to carrying out a thorough investigation as soon as possible.”

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Police said the parents of the babies were being kept fully updated by specially trained officers as the investigation proceeds.

“This is an extremely difficult time for all the families and it is important to remember that, at the heart of this, there are a number of bereaved families seeking answers as to what happened to their children,” Cheshire Police said in a statement.

Neil Fearn, the chief executive of Pryers Solicitors, the firm representing the families of two babies who died at the hospital, said: “We are hopeful that the investigation can provide answers for the families of these children.