Selective abortion in favour of males has left China with 32 million more boys than girls, creating an imbalance that will endure for decades, an investigation released on Friday warned.
The probe provides ammunition for those experts who predict China's obsession with a male heir will sow a bitter fruit as men facing a life of bachelorhood fight for a bride.
"Although some imaginative and extreme solutions have been suggested, nothing can be done now to prevent this imminent generation of excess men," says the paper, published online by the British Medical Journal (BMJ).
In most countries, males slightly outnumber females -- between 103 and 107 male births for every 100 female births.
But in China and other Asian countries, the sex ratio has widened sharply as the traditional preference for boys is reinforced by the availability of cheap ultrasound diagnostics and abortion.
This has enabled Chinese couples to use pregnancy termination to prevent a female birth, a practice that is officially condemned as well as illegal.
In China, an additional factor has been the "one-child" policy.
In general, parents who have a second child are liable to pay a fine and contribute disproportionately towards the child's education.
But in some provinces, a second child is permitted if the first is a girl or if parents are experiencing "hardship." And in a few others, couples are allowed a second child and sometimes a third, regardless of sex.
In the paper, Zhejiang university professors Wei Xing Zhu and Li Lu and Therese Hesketh of University College London found that in 2005 alone, China had more than 1.1 million excess male births.
Among Chinese aged below 20, the greatest gender imbalances were among one-to-four-year-olds, where there were 124 male to 100 female births, with 126 to 100 in rural areas, they found.
The gap was especially big in provinces where the one-child policy was strictly enforced and also in rural areas.
Jiangxi and Henan provinces had ratios of over 140 male births compared to female births in the 1-4 age group.
Among second births, the sex ratio was even higher, at 143 males to 100 female. It peaked at a massive 192 boys to 100 girls in Jiangsu province.
Only two provinces -- Tibet and Xinjiang, the most permissive in terms of the one-child policy -- had normal sex ratios.
"Sex selective abortion accounts for almost all the excess males," the paper said. "(...) Enforcing the existing ban on sex selective abortion could lead to normalisation of ratios."
Other policy options are to loosen enforcement of the one-child policy so that couples can have a second child if the first child is a girl, it said.
The paper does not deal with the social consequences of the extraordinary imbalance, but suggests there are rays of light.
Since since 2000, the government has launched policies aimed at countering the imbalance, with a "care for girls" awareness campaign and reforms of inheritance laws, it says.
Partially as a result, the sex ratio of birth did not change between 2000 and 2005, and in many urban areas, the ratio for the first and usually only birth is now within normal limits.
The figures come from a mini-census in China in 2005, covering one percent of the population, that sought to rectify flaws in a 2000 census. A total of 4.764 million people under the age of 20 were included in the study.
In a commentary, Tao Liu and Xing-yi Zhang of Jilin University said preferences for sons in China were starting to erode with urbanisation and industrialisation.
Social systems, pensions and higher standards of living eased the son's traditional role of caring for his parents.
China could also follow the lead of South Korea, he said.
In 1992, South Korea had "an astounding" 229-to-100 gender imbalance, which prompted it to launch a public-awareness campaign combined with strictly enforced laws on gender selection. By 2004, there were 110 male births to 100 females.