The eggs, being referred to in Chinese media and on the internet as "rubber eggs" or "ping pong eggs," are too hard to eat, raising suspicion they are fake, after appearing in "small numbers" in markets nationwide, Xinhua news agency said.

"The investigation is designed to appease consumers' concerns, after some suspected they bought artificial eggs made by unconscientious traders seeking profits," it reported.

However, the eggs' hardness could be a natural occurrence, caused by hens consuming large amounts of food enriched with a compound called gossypol, which binds to protein in egg yolks, Xinhua said.

"While gossypol normally exists in the residue of cotton seeds added to chicken feed as an extra protein source, large doses of the compound will suppress sperm activity as gossypol has been tested to be used in male contraceptive pills," it added.

Food safety worries are nothing new to China, where tales of fake cooking oil, tainted milk and watermelons which explode from being fed too much fertilizer regularly appear in the news.

In 2008, at least six children died and nearly 300,000 became ill from powdered milk laced with melamine, an industrial chemical added to low quality or diluted milk to fool inspectors by giving misleadingly high readings for protein levels.

(Reporting by Sally Huang and Ben Blanchard; Editing by Sanjeev Miglani)

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