BEIJING  A bias in favor of male offspring has left China with 32 million more boys under the age of 20 than girls, creating “an imminent generation of excess men,” a study released Friday said.

For the next 20 years, China will have increasingly more men than women of reproductive age, according to the paper, which was published online by the British Medical Journal. “Nothing can be done now to prevent this,” the researchers said.

Chinese government planners have long known that the urge of couples to have sons was skewing the gender balance of the population. But the study, by two Chinese university professors and a London researcher, provides some of the first hard data on the extent of the disparity and the factors contributing to it.

In 2005 , they found, births of boys in China exceeded births of girls by more than 1.1 million. There were 120 boys born for every 100 girls.