Parents are being warned to keep poorly children at home after a sickness bug hit a primary school.

St John’s Stonefold in Rising Bridge has been struck by an outbreak of gastroenteritis, which headteacher Stephen Oldfield said has ‘affected an unusually large number of people’. He has issued a letter to parents asking them to keep youngsters with sickness and diarrhoea away from school for 48 hours to avoid the infection spreading further.

He said they have had a ‘high absent rate’ of both staff and pupils and have introduced extra cleaning measures to reduce risk of infection.

More than 20 pupils are believed to have been affected and the letter explains: “We currently have a sickness bug in school and are doing all we can to ensure this is eradicated.

“If your child is physically sick or has diarrhoea for whatever reason please ensure he/she remains at home for 48 hours from the onset of the condition.

“We have a high absent rate among both staff and pupils at present due to the condition, and anything you can do to help in this respect will be greatly appreciated.

“We have been encouraging careful hand washing and have been using anti-bacterial gel at lunchtime. Hopefully if we all work together the problem can be tackled quickly.”

Karen Lamminen, of Public Health England, said the outbreak at St John’s Stonefold has not been reported to the authority, but that norovirus is rife at this time of year.

She added: “Norovirus is circulating in the community. That’s not unusual, because it is a winter bug.

“It is highly contagious and spreads rapidly in closed environments such as hospitals, schools and care homes.

“It can be spread through contact with the infected person, by contact with contaminated surfaces or objects, or by consuming contaminated food or water.

“In somewhere like a school it can spread like wildfire.

“We have advice we can issue to schools - mainly that children need to stay home for 48 hours - if they come back to early it will just spread.

“Norovirus infection is a self-limiting illness and you will usually recover naturally without treatment. It is, however, important to take plenty of drinks to replace lost fluids.”