He said there were no more tainted products on shop shelves and those allegedly involved in the scandal had been charged.

"I promised that I would not stay on after all my responsibilities had been met so I announce my resignation and return to my neurosurgery post and medical education career," he said.

Earlier, the Pingtung Prosecutors Office announced that eight people, including bosses of Taiwanese cooking oil company Chang Guann, had been formally charged with supplying cooking oil contaminated with "gutter oil", a product recycled from food waste and leather processing.

Yeh Wen-hsiang, head of the Kaohsiung-based supplier, his deputy Tai Chi-chuan and a third employee named Shih Min-yu were charged with violating food safety regulations and fraudulently selling contaminated cooking oil to 235 companies this year, prosecutors said.

Kuo Lieh-cheng, the owner of an illegal edible oil recycling factory that supplied Chang Guann, was accused of the same offence.

Chang Guann allegedly blended the gutter oil with regular cooking oil and lard oil - made from pork fat but intended for use in animal feed or for industrial use - and sold the final product as cooking oil to food firms in Taiwan and Hong Kong.

Three executives from animal feed manufacturer Ching Wei, as well as a senior staff member of another feed firm, were charged with illegally recycling waste oil.