By Lindsay Beck

BEIJING, April 4 (Reuters) - Women and children in China face a growing threat of being trafficked and sold into marriage or sex work, as labour migration and a widening gender imbalance put them at risk, an international aid group said on Wednesday.

About 119 boys are born in China for every 100 girls, an imbalance that has grown since Beijing introduced a one-child policy more than 25 years ago that has bolstered a traditional preference for boys, resulting in abortions of female foetuses and abandonment of baby girls.

"Lack of girls for marriage in the eastern and rural areas is fuelling a demand for girl babies to be raised as future brides for better-off farmers' sons," Kate Wedgwood, China country director for Save the Children, told the Foreign Correspondents' Club.

The strict family-planning policy means China will be home to 30 million more men of marriageable age than women by 2020, state media has reported.

Wedgwood said it also meant many poor, rural families were reluctant to register children born outside of the plan, leaving them vulnerable to trafficking or exploitation by local officials who encourage them to hand over their babies in return for being excused a fine.

Migration for employment was also leaving many vulnerable to traffickers, both rural workers who move across the country to huge urban centres, and children who are left behind, often without adequate care.

"The number of women and children from poor and underdeveloped regions who migrate due to employment and or business considerations, but end up being abducted or trafficked, will continue to grow," Wang Jinling, a researcher at the Zhejiang Academy of Social Sciences, said in a paper.

"Some of them will be sold and married, and some will be forced to become offenders, such as sex workers and drug traffickers," Wang wrote.

Wedgwood said her organisation was working with the government on projects in rural border areas to register job recruiters to ensure their legitimacy and to make "safe migration classes" mandatory for villagers seeking work in urban areas.

A lack of reliable figures made it nearly impossible to track trends in trafficking, she added.

The Ministry of Public Security recorded 2,500 cases of trafficking in China in 2006, but that figure only includes resolved cases, rather than those reported, and fails to make clear whether "cases" involve individuals or rings.

But Chinese media regularly report on abduction rings.

One report cited police in the southern city of Dongguan -- a major manufacturing hubs and home to millions of migrant factory workers -- as saying baby boys in the area could be sold for 10- 20,000 yuan ($1,250-$2,500), and girls for a few thousand.