China will have 30 million more men of marriageable age than women by 2020, making it difficult for them to find wives, according to a national report.
Poorly-educated men may increasingly struggle to find wives
The gender imbalance could lead to social instability, the report by the State Population and Family Planning Commission warned.
It found that around 118 boys were born to every 100 girls in 2005.
A traditional preference for boys, in a country with a one-child policy, is the root of the problem, the report says.
Abortions on female foetuses are believed to be widespread as couples, particularly in rural areas, hope for a son who will look after them in their old age.
There is also suspected under-reporting of female births.
The report said the 118 to 100 ratio of newborn boys to girls had jumped from 110 to 100 recorded in 2000.
In some areas of southern China, such as Guangdong and Hainan, the figure was 130 boys to 100 girls in 2005.
Nationwide this means there will be 30 million more men than women by 2020, making it difficult for those particularly with low income or little education to find a wife, the report said.
"The increasing difficulties men face finding wives may lead to social instability," the report said.
The report went on: "We need to develop a 'movement to embrace girls'... and effectively contain the trend towards greater gender imbalances."
The report also said that China's current 1.3 billion population would grow by 200 million by 2033.
And the number of 60-year-olds and over will jump from the current 143 million to 430 million by 2040, 30% of the total population.
The country will need to improve its social security and retirement system, as well as its family planning policies and education and health services, the report concludes.
The BBC's Daniel Griffiths, in Beijing, says the one-child policy has already led to other major problems, with many single children facing the prospect of supporting two parents and four grandparents well into their old age.