The student, surnamed Chang and a freshman at the China University of Mining and Technology based in Xuzhou, has been arrested by local police.

According to Zou Fangming, deputy party chief of the university, Chang bought 250 grams of the virulent chemical on May 22 and poured it into the cups of three classmates when they were having supper together at the campus canteen on May 29.

One of the three victims was admitted to hospital in north China's Hebei Province on June 8 after returning home ill. By that time he was barely able to walk on his own and was suffering severe stomach cramps, vomiting and chest pain.

The three students were later confirmed to have been poisoned by thallium, according to tests carried out by the China Disease Control and Prevention Center on June 10.

Police in Xuzhou ruled out the possibility that the food in the canteen was contaminated and believed somebody must have poisoned the victims' supper. They tracked down Chang who confessed to deliberately poisoning his classmates.

Chang, from the material science engineering department, said the four students used to be close friends but he had been excluded from the group.

According to his confession, he decided to let them "taste the torture of being poisoned," said police. Chang said he diluted the thallium in a bottle of water. At suppertime he put the contaminated water in the three students' cups when they were away from the table.

Chang's classmates said he was a sensitive loner and that he often felt maltreated by his classmates.

The three victims are still being treated in Chaoyang Hospital in Beijing and are in a stable condition.

Police are continuing their investigations.