Post-mortem tests on 22 children who died after eating a school lunch in India have confirmed that insecticide was in the food or cooking oil, doctors said.

Patna Medical College said results of chemical analysis of the ingredients seized from the school are still pending.

The free meal was served to the children in Gandamal village in Masrakh block, 80 50 miles north of Patna, the Bihar state capital.

The 25 children and school cook still being treated in hospital are unlikely to suffer from any serious after-effects of eating the tainted food, said Patna Medical College hospital superintendent Amarkant Jha Amaralso.

It is not clear how chemicals ended up in the food. One official said it might not have been properly washed before it was cooked.

The children, between the ages of five and 12, fell ill on Tuesday, soon after eating the meal of rice, lentils, soybeans and potatoes.

The lunch, part of a popular national campaign to give at least one daily hot meal to children from poor families, was cooked in the school kitchen.

The children were rushed to a local hospital and later to Patna for treatment.

Authorities suspended an official in charge of the free meal scheme in the school and registered a case of criminal negligence against the school head teacher, who fled as soon as the children fell ill.

Angry villagers, joined by members of local opposition parties, closed shops and businesses near the school and overturned and burned four police vehicles.

A preliminary investigation yesterday suggested the food contained an organophosphate used as an insecticide on rice and wheat crops. It is believed the grain was not washed before it was served at the school, he said.

However, villagers said the problem appeared to be with a side dish of soybeans and potatoes, not grain. Children who had not eaten that dish were fine, although they had eaten the rice and lentils, several villagers said.

India's midday meal scheme is one of the world's biggest school nutrition programmes. State governments have the freedom to decide on menus and timings of the meals, depending on local conditions and availability of food rations. It was first introduced in southern India, where it was seen as an incentive for poor parents to send their children to school.

Since then the programme has been replicated across the country, covering 120 million schoolchildren. It's as part of an effort to address concerns about malnutrition, which the government says nearly half of all Indian children suffer from.