Qing's story, highlighted in a BBC documentary, is all too common in China, where "wives", mostly young girls from rural areas, can be acquired for between about £500 and £2,000. Despite a government crackdown on bride-trafficking that has seen some 42,000 women rescued over the past four years, many more are never found.

The resurgence in the trade in wives comes as China faces an acute lack of women. So serious is the dearth of females that it is estimated that by 2020, 40 million men will be forced to remain bachelors.

The shortage of women has the government scrambling to redress the balance. Under the Care for Girls programme, financial incentives are being offered to families who have daughters, helping them to pay for education and medical costs. Last week it was announced that female astronauts are to be recruited for China's space programme, in an effort to create positive female role models and reverse the traditional Chinese preference for boys over girls.

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"It dates back to feudal society," said Professor Zhu Chuzhu of Xi'an Jiaotong University, who has been studying China's gender imbalance since the mid-1990s. "The idea that boys are more important than girls has been rooted in people's minds for thousands of years. It's difficult to change that culture, especially in rural areas where people are less educated."

The prospect of tens of millions of lonely, disgruntled single men, known in China as guang guan, or "bare branches", is alarming the government, which fears the effect they will have on social stability. "It will create a potential threat to national security," said Professor Zhu. "Too many men increases the likelihood of sexual crimes occurring. It will make the existing phenomenon of prostitution and the selling of women much worse."

China's last census revealed that 117 boys are born for every 100 girls. In some parts of Guangdong and Hainan provinces in southern China, the ratio is as high as 135 boys to 100 girls. The introduction of the "one-child" policy in 1979, designed to curb the growth of China's vast population, has only exacerbated the problem. Pregnant women in rural China took advantage of new, portable ultrasound scanning machines to ascertain the sex of their baby in advance: in return for money, nurses and doctors smile if the foetus is a boy, or frown if it's a girl. Selective abortions remain common, despite being outlawed in 1995.

For single men in the countryside, the lack of women is devastating. Poorly educated (97 per cent of men in rural areas fail to finish school and some 40 per cent are illiterate), they can either stay in villages where single women are scarce, or join the millions of migrant workers who flock to the cities to work on construction sites in the hope of making enough money to make them attractive marriage prospects.

Chinese women, though, are becoming more choosy about who they marry. Many women from the country have found work in the factories of boom towns such as Shenzhen, and are reluctant to return to a hand-to-mouth existence on a small farm in the middle of nowhere. In the big cities, a new generation of well-educated, independent women has emerged.

"Jane", an engineer with a master's degree, is one of them, and is in no rush to get married. "I don't think I am very picky, but my parents think I am," she laughs. "I think I require very basic things: a man with a good personality who treats his parents well and who is well educated with a good job. If he doesn't have a good job, then it's not going to work. I would like to get married, but if I can't find the right man, I won't. I'll just treat myself very well."

For China's growing army of bachelors, that's an option most of them won't have.

'Looking for China Girl', is on Tuesday at 9pm on BBC2