Some products recalled during the 2008 outbreak that killed six and sickened 300,000 are thought to have been put on the market again, according to state media.

Health ministry officials said the probe would “thoroughly check potential problems in food safety”, said one report. "There are still some businesses and individuals that ignore the safety and health of the public and are blinded by greed.”

Companies involved in new melamine discoveries have been based in various provinces of the country, including Shanghai, Guizhou and the northeastern province of Liaoning.