A PRIMARY school was forced to close yesterday after more than 80 children and seven staff fell sick with a mystery bug.

All of those who became unwell at Blackhall Colliery Primary School, in County Durham, were suffering from vomiting.

Last night, a spokesman for the local education authority, Durham County Council, said the reason the school had been shut until Monday was because of the number of staff members struck down.

The spokesman said: "About 80-plus children and a number of members of staff have gone down with a vomiting illness, and with so many staff ill it was impractical to keep the school open."

A spokeswoman for the Health Protection Agency (HPA) said it was working with the school and environmental health officers to try to identify the source of the infection.

"At the moment, we don't know what the source is, but early indications are that it is a viral infection rather than anything to do with food or hygiene," she said.

Dr Deborah Wilson, also of the HPA, confirmed that about 80 children and seven staff were taken ill. She said the source remained a mystery, but added: "What we can say is that the school let us know yesterday that an unusually large number of children were vomiting.

"We gave them the usual advice over the symptoms and advice on preventing the infection spreading."

She said the numbers were higher than was usual in such an outbreak.

"Early indications are that it is a viral infection known as winter vomiting virus, rather than a food poisoning outbreak or anything more serious than that."

Dr Wilson said she was talking to local GPs and that environmental health staff would take samples from some of those who had contracted the virus.

She said: "It often affects nursing and residential homes and office environments. It is very infectious and we see it every year."