BEIJING  -- China and the Republic of Korea are to jointly investigate how Chinese frozen eel exports came to contain carcinogenic malachite green, a fungicide banned in food production in China, according to a statement from China's quality watchdog on Friday.

Chinese investigators had found no information about the producer, identified by the Republic of Korea media as Jiangxi Yichun Eel Industry Development Co. Ltd, said the statement from the General Administration of  Quality Supervision, Inspection and Quarantine (GAQSIQ).

The producer had not registered with China's import and export inspection and quarantine departments, nor was it an authorized foreign trade dealer whose qualifications should have been registered with commerce departments, it said.

Local fishery industry associations also failed to identify the company during the preliminary investigation, which was started immediately after the Chinese side learned about the incident on Wednesday night and involved the ministries of commerce and agriculture and the GAQSIQ.

"This appears to be an illegal export. As the tainted products were discovered in retail sales, the Chinese side will look into the case with the Republic of Korea and release the investigation results in a timely way," it said.

The use of malachite green, which can cause cancer, is prohibited in many parts of the world. China banned fish farmers and other businesses from using the chemical as fungicide in 2002.

This is the second overseas food quality concerning Chinese-made food in two months following the discovery of tainted frozen dumplings, which left at least 10 people ill in Japan.

In the past two weeks, the dumpling producer, Tianyang Food Plant, in north China's Hebei Province, has been investigated by both Chinese and Japanese authorities, but no problems have been found in production.