A hospital health care assistant has been arrested by police investigating the "unexpected" deaths of four children on a paediatric ward at a hospital in Berkshire.

The unnamed woman, who worked on ward 24 at the Wexham Park hospital in Slough, was questioned by officers from Thames Valley police after consultant paediatricians became concerned about the number of deaths on the ward.

Four children, who were all being treated for non life-threatening illnesses, have died on the ward in the last eight months, compared with the usual average of two or three unexpected deaths a year.

The hospital yesterday refused to confirm if the health care assistant, who has been released on police bail pending further inquiries, was the last member of staff with the children before they died or whether they shared the same cause of death.

Margaret Edwards, chief executive of the Heatherwood and Wexham Park Hospitals Trust, said the deaths could all just be "coincidence".

"Taken in isolation all these deaths don't give us any reason for concern. Concern has been raised because we experienced more deaths than we expected.

"It may just be a statistical cluster, a coincidence. But we have taken it seriously and asked the police to investigate."

Ms Edwards said the circumstances surrounding the death of each child had been reviewed at the time as a matter of routine and nothing suspicious had been found. Postmortem examinations to determine the cause of death proved inconclusive.

The hospital and the police refused to reveal the identities of the children to protect their families. But the coroner, Peter Bedford, confirmed the last alleged victim, who is understood to have died very recently, was a baby.

Ms Edwards said she had first been alerted to the fact there might be a problem on Monday night when consultant paediatricians who work on the ward had gone to see her and expressed concern at the number of unexpected deaths. Thames Valley police were called in, and on Wednesday evening they arrested the nursing assistant. She has also been suspended.

Detective Superintendent Steve Morrison, of Thames Valley police murder squad, said the parents of the children, who were all aged under 16, were distressed by news of the arrest and investigation.

"Clearly, they are very concerned to see that the truth of what has happened is established. We can sympathise with how traumatic this must be for them."

A family liaison officer had been drafted in to to keep the families informed about the progress of the investigation.

Mr Morrison confirmed his officers would be working closely with other constabularies, including Essex, which investigated a nurse over similar allegations last year. He said there were no immediate plans to exhume bodies of any buried children.

"This is a very early stage in the inquiry. Clearly, a number of people will be interviewed immediately and we will interview anyone who can help us with our inquiry.

"It will take weeks and months to get a clear picture of what is happening."

Mr Morrison emphasised that all the deaths had occurred on ward 24, which is staffed by 40 nurses and cares for up to 30 children. He said none of the trust's other wards or children's services were under investigation.

An incident room has been set up at Maidenhead police station and 15 detectives have been assigned to investigate.

Staff at the hospital, which is in parkland on the outskirts of Slough, were said to be "very distressed" over the allegations about their colleague.