'The ministry has demanded that the Hubei food safety supervision task force investigate and relevant authorities conduct laboratory tests on samples from that brand,' he said.

Local food safety authorities had earlier refused a parent's request to investigate the formula made by Nasdaq-listed Synutra, saying they did not conduct tests at consumers' behest, state media reported.

Medical tests indicated the levels of hormones in three Wuhan girls who were fed the same baby formula exceeded those of the average adult woman, the China Daily reported on Monday.

A fourth case was reported in Beijing, the Beijing Times reported. The one-year-old girl had been breastfed until being bottle fed with Synutra two weeks ago.

The ministry said medical experts were also assisting a separate medical investigation into the cause of the infants' condition and the result would be announced to the public soon.

Early development of breasts is the most common symptom of premature puberty in girls but in most cases, Deng said, a link could not be found with food or the environment.

Synutra said its products were safe and that no man-made hormones or illegal substances had been added during production.

It also said it was preparing to sue Hong Kong's Phoenix TV, which ran the story, for 'fabricating lies and smearing a famous Chinese brand'.

In Guangzhou, many hospitals have stopped supplying Synutra formula for newborns at the request of parents, who are insisting that foreign brands be provided instead.

Wang Shuhuai, a researcher from the China Institute of Veterinary Drug Control, said a standard for traces of hormones in dairy products had yet to be released. Only two hormones were approved by the Ministry of Agriculture to be used on cows in mating season and adding those hormones during milking season would reduce milk production.

Chinese dairy products were recalled worldwide in 2008 after it was found that melamine, an industrial additive, was widely added to raw milk to increase nitrogen levels and get past protein tests. At least six children died from kidney failure and 300,000 others suffered from kidney stones caused by the melamine, which was found in the products of 22 Chinese dairy firms.

Despite repeated pledges by the authorities to crack down on melamine-tainted milk, it is a problem that keeps resurfacing. In July, authorities seized 76 tonnes of tainted milk powder, which was traced back to a dairy factory in Qinghai . In February, hundreds of tonnes of stored toxic milk powder was discovered. Some of it was sold to make dairy products such as milk candy.

Premature puberty

The four babies who reportedly began growing breasts after drinking Synutra formula were aged, in months, up to: 15