China introduced a one-child policy to control its soaring population

There were 163.5 boys for every 100 girls aged up to four in the eastern city of Lianyungang, it said.

Abortions of female foetuses were the main cause of the imbalance, CFPA said.

China's one-child policy, and a preference for male heirs, has led some women to abort girls despite a ban on abortions after a baby's sex is known.

The ban does not apply if an abortion is required for medical reasons.

Social instability

Ninety-nine cities had gender ratios higher than 125, state-run news agency Xinhua quoted the CFPA as saying in a report. The provinces of Hainan, Henan, Guangdong and Anhui also had particularly high imbalances, it said.

China's national average was 119 in 2005 but the UN recommends a gender ratio of no more than 107.

Song Jiang, from Renmin University told Xinhua that "the traditional thinking that boys are better than girls" was behind the imbalance, particularly in poor areas.

The report also warned of China's ageing population. More than 11% of the population was older than 60, with that figure expected to grow to 16.1% by 2020.

In January, the State Population and Family Planning Commission warned that gender imbalances could lead to social instability.

It said China's men were facing a shortage of wives, with a predicted 30 million more men of marriageable age than women by 2020.

Correspondents say the one-child policy has caused other social problems, with many single children facing the prospect of supporting two parents and four grandparents well into their old age.

China established the policy in the late 1970s to control its soaring population.