The poisonous eggs have been found in Wumart supermarkets in the capital of China, with more than 10,000 kilograms already sold, a local franchiser said.

The authority has traced the source of the eggs to Baiyangdian City, in Hebei Province, a place famed for its high quality duck eggs with deep orange yolks.

Reports said farmers at the duck farms in Shijiazhuang, capital city of the province, gave their ducks feed that contained Sudan IV, a banned ingredient in food that can cause cancer.

These farmers are among the main duck egg providers of wholesale markets and supermarkets in Beijing, the report said.

The relevant authority in Shanghai hasn't received any report on the issue, and the duck eggs for sale in the Wumart supermarkets in Shanghai are all from the city's neighboring Jiangsu and Zhejiang provinces.

"We haven't received any notice about these eggs," said an employee from a Wumart supermarket in the city's north.

Sudan-1 was once found in a vegetable used in KFC's soup last November. Though KFC suspended the sale of the soup, the potentially poisonous element has caused widespread public concern.