The imbalance is a consequence of China's "one couple, one child" family planning policy, meant to slow runaway population growth, The Times of London reported Monday.
The cultural preference for boys is grounded in a desire to carry on the family line and a traditional tendency not to value girls, the report said. Ultrasound scans, introduced in the late 1980s, resulted in an increase of abortions of female fetuses, The Times said.
The normal ratio of 103-107 boys born for every 100 girls started to shift -- from 108 boys in 1982 to 111 in 1990, and 116 a decade later.
A report in the British Medical Journal said poorer, largely rural provinces have the highest imbalances in the country, with 140 boys for every 100 girls in the 1-to-4 age range.
"Nothing can be done now to prevent this imminent generation of excess men," Dr. Therese Hesketh, author of the report, said.