Beijing authorities also said that China's biggest producer of powdered milk had known for months that its baby formula was tainted with the industrial chemical melamine. There were complaints about infant formula sold by the Sanlu Group Co. as early as December, 2007, China Central Television reported, citing an investigation by the State Council, China's Cabinet.

"During these eight months, the company did not inform the government and did not take proper measures, therefore making the situation worse," CCTV said.

Melamine, which can cause kidney stones and kidney failure in babies, has since been found in infant formula and other milk products from 22 of China's dairy companies.

Baby formula and other milk products have been pulled from stores around the country and Chinese goods including liquid milk, instant coffee mix and milk-based candy have been banned around Asia.

Hong Kong parents took their children for health checkups Monday after the government announced that a 3-year-old local girl who drank contaminated Chinese milk had been diagnosed with a kidney stone. She was discharged from the hospital, the Hong Kong government said.

Stricter monitoring urged
The resignation of Changjiang, who headed the General Administration of Quality Supervision, Inspection and Quarantine since 2001, comes a year after he and the government promised to overhaul the system.

New regulations and procedures were introduced in an attempt to restore consumer confidence and preserve export markets after a string of recalls and warnings abroad over tainted toothpaste, faulty tires and other goods.

Yet the latest crisis indicates problems were still slipping through the cracks.

On Monday, the World Health Organization urged stricter monitoring of the industry.

Sanlu and several other leading companies found to have produced tainted milk had been given inspection-free status by China's product quality watchdog.

That privilege has since been rescinded, but WHO China representative Hans Troedsson stressed it was only a first step and that quality issues can crop up at any point in the supply chain, from the farm to the store.