Chinese and Hong Kong authorities have recalled tainted products
The head of China's quality watchdog has resigned amid a growing scandal over melamine-contaminated milk, the official Xinhua news agency reports.
Li Changjiang stepped down "with the approval of the State Council", China's cabinet, Xinhua said.
Earlier Prime Minister Wen Jiabao apologised for the contamination, which has made nearly 53,000 children ill.
Four children have died and about 13,000 remain in hospital, 104 of them reportedly in a serious condition.
The problem was first revealed two weeks ago, when milk powder from the Sanlu Group was found to contain melamine, an industrial chemical.
World Health Organisation urges breastfeeding
At least 22 other companies have since become involved in the scandal, and milk products made by the Yili, Mengniu and other groups have been recalled from supermarket shelves in China and many other countries.
Estimates of the number of people affected have jumped in recent days, probably as the result of checks by health officials, who looked at hospital records from May this year to trace the origins of the contamination.
Mr Li, whose agency is responsible for ensuring that China's food supply chain is safe, will be replaced by Wang Yong, a senior cabinet official, Xinhua said.
Prime Minister Wen has promised to reform the dairy industry and to improve its regulation - and this effort appears to have begun with the resignation of Mr Li, says the BBC's James Reynolds in Beijing.
I sincerely apologise to all of you. What we are doing now is to ensure that nothing like this will ever happen again
Wen Jiabao Chinese premier
He is the first central government official to lose his job over the scandal. His resignation follows the sacking of a number of local officials and the detention of several dozen people involved in the supply of contaminated milk.
Touring hospitals in the capital, Mr Wen said he felt "extremely guilty" about the health scandal.
"I sincerely apologise to all of you," he said, after visiting sick children in hospital.
"What we are doing now is to ensure that nothing like this will ever happen again, and we are not only talking about milk. We will never let the same situation repeat with any kind of product."
But parents queuing outside the hospitals to get their children checked for kidney stones told the BBC they were still not sure what they could safely feed their children.
One father said that he believed the people who contaminated the milk had no conscience.
"It's unbearable," he said.
Meanwhile, the Chinese Ministry of Agriculture said despairing farmers were dumping milk and killing cattle after companies stopped buying their supplies, reported Reuters news agency.
It promised subsidies to help farmers.
Children are checked in hospital
Anger and fear
Correspondents say the scale of this contamination scandal is only now becoming clear.
Tens of thousands of children are now known to have been affected in China itself.
Meanwhile authorities in Singapore, Malaysia, Hong Kong, Taiwan, Japan, the Philippines, Vietnam and elsewhere have been testing Chinese-made dairy products or pulling them from the shelves altogether.
In Hong Kong, the two main supermarket chains, Wellcome and Park'n'Shop, are now recalling products made by Nestle, Dutch Lady and Mr Brown.
Anxious parents rushed their children to health centres for check-ups amid reports that two Hong Kong residents, one a three-year-old girl, were suspected to be suffering kidney stones after drinking milk from mainland China.
Melamine is used in making plastics and is high in nitrogen, which makes products appear to have a higher protein count. Health experts say that ingesting small amounts does no harm, but sustained use can cause kidney stones and renal failure, especially among the young.
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