They decided to suspend all classes today - affecting 1,869 pupils - based on the advice of department doctors. But a spokesman for the bureau stressed last night that the children were not suffering from a mystery virus that is causing panic in Shenzhen.

'Those infected were suffering what is called an enteric viral infection, which is basically a viral infection that has entered the intestines,' he said.

'The virus is probably spreading because of the cool weather and the close interaction between children at the primary school.'

Chinese University microbiology professor John Tam Siu-lun said the infection could be caused by a number of possible viruses, including the noro virus, which can spread in confined environments such as schools, or through contaminated food.

The father of a Primary One pupils at the school said his daughter started showing symptoms on Wednesday.

'A pupil was sick late last week and all week more got sick, but they are only taking action now. I am a bit disappointed by the slow reaction,' he said.

The bureau spokesman said the Health Department recommended the school increase hygiene measures, such as asking the students to wash their hands frequently, when classes resume on Monday. 'Not all the absent students were ill. Some of them did not come to school as a precaution,' the spokesman said.

The affected pupils experienced vomiting and severe diarrhoea, he said, but not the rash that has been associated with the virus reported on the mainland.

Health Department officials will be there to advise when the school is cleaned and disinfected today.