The authorities on the island said that the eggs contained twice the legal limit of melamine, an industrial chemical which made over 50,000 infants ill and killed four when it was discovered in powdered baby milk over the summer.

Melamine, which is more commonly found in plastics, was added by unscrupulous traders to "bulk up" milk and make it appear richer in protein.

However, the chemical triggers the formation of kidney stones.

Authorities in Hong Kong said they have contacted Beijing to ask for an investigation after melamine was discovered in eggs from China's biggest producer, Hanwei, a company in the northern town of Dalian. Hanwei said it is investigating.

"We have contacted the mainland's food safety agency and hope they can do more to reduce the risk at the source," said York Chow, the island's health secretary.

A food safety inspector in Dalian said that eggs tainted with melamine were detected last month and were destroyed, but that tests this month showed no traces of the chemical.

Wen Jiabao, the Chinese prime minister, promised last Saturday that China would do all it could to bring its food quality standards up to international levels.

China has not updated the number of children affected by melamine poisoning since the third week of September, and there are indications that the official number may be far too low. Health officials said on Sunday that one quarter, or 75,000, of the 300,000 families in Beijing with a child of less than three had been affected.

Meanwhile, Hong Kong has started testing meat and vegetables coming from the mainland. One theory is that the chickens may have eaten feed contaminated with the chemical.

It has also emerged that cyromazine, a derivative of melamine, is widely used in pesticides and animal feed. This could have been absorbed and pass upwards through the food chain.

"As we have found melamine in eggs, we shall also test chicken meat and we shall also look at offal, for example chicken kidneys and pig kidneys," said Mr Chow.

Batches of baby milk produced before melamine testing became standard have only recently been recalled from Chinese supermarket shelves.

The Chinese media has also sought to play down the possibility of toxic eggs. There were no reports of two incidents in Japan and South Korea when eggs laced with melamine were destroyed. South Korea ditched 23 tonnes of egg powder from China on Oct 22 after discovering the chemical.

Han Wei, the founder of the Dalian Hanwei company, is the vice-chairman of the city's federation of Industry and Commerce and a notable party delegate.

In Japan, a major food producer announced a recall of 2.7 million pizzas and packs of sausages after it discovered three times the government limit of cyanide in its water supply. Itoham Foods put advertisements in national newspapers apologising for the contamination. Last Friday, the country's largest maker of instant noodles, Nissin, recalled its Cup Noodle line after a woman was taken ill with insecticide poisoning.