Industrial and commercial authorities are checking markets, inspecting vendors' certificates to ensure that food and the amounts are listed and the dates purchased and sold are correct.

Market stallholders have to sign an agreement accepting responsibility for the food and agreeing to disciplinary procedures if found wanting.

One stallholder, named Huang, at the Cao'an Grocery Wholesale Market in Putuo District said the new rules were "severe," but he would go along with them.

"We now have to import vegetables only from suppliers who have good reputations, otherwise we will not be relaxed about the quality of food we sell," said Huang.

Shen Peimin, manager of the market's vegetable department, said the market started to enforce the new rules last Saturday.

"We knew the vendors might complain but they have to obey the law," he said.

He said the new rules have made it easier to trace the origin of the food supply if there are problems.

"Seventy percent of the vegetables in the local market are from other provinces," said Shen. "If we find problems like traces of pesticide in the products we'll report this to the local food authority where the vegetables come from and if the problem occurs again we shall refuse to buy produce from that area."