Medical experts confirmed on Tuesday that 300 primary school pupils who were sent to hospital after eating lunch at their school on the first day of the new semester in southwest China, are suffering from food poisoning.

It remains unclear whether the food was contaminated by the food seller or during its preparation at the school, said Fu Xiaolu, director of the Disease Prevention and Control Center of Chengdu, capital of Sichuan Province.

About 300 pupils at the Chongzhou City Experimental Primary School began suffering from headache, fever, vomiting, stomach-ache and diarrhoea on Saturday and Sunday and were hospitalized. They ate lunch at the school on Friday, their first day back at school after the summer vacation.

About 200 students are still in hospital. No deaths have been reported.

Vomiting and diarrhea usually develop six to 24 hours after a person eats food contaminated by the Shigella dysenteriae which experts say they found in the food. The pupils stricken by the bacteria showed symptoms 30 to 40 hours after their lunch, said Fu.

It generally takes a week for a person to recover after being sickened by the bacteria.

Disease control and sanitary inspection experts led by officials from the Sichuan provincial health department are investigating and helping with the treatment of sickened pupils.

The school suspended classes on Monday. School sources said they would decide when to restart classes after two thirds of the pupils are recovered.