China is planning to tighten punishments for sex-selective abortions amid concerns that its widening gender imbalance will lead to wife trafficking, sexual crimes and social frustration.

Shocking new figures released by the state media show that the worst affected city, Lianyungang in Jiangsu province, has a ratio of 165 boys to 100 girls among children aged one to four.

Nationwide, six males are born for every five females, far above the international average. With the gap growing every year as a result of increased access to ultrasound sex-checking technology, one senior official warned that China faces the 'most serious gender imbalance in the world'.

There has been alarm for more than a decade, but it has reached a new pitch in recent months as state demographers forecast that 37 million men will be unable to find wives by 2020. Already there are 18 million more men than women of marriageable age. In a recent survey by the China Youth Daily, 85 per cent of respondents were worried about the implications of the gender gap.

The main reason for the imbalance is a traditional preference for boys, particularly in the countryside. Males are considered better at carrying on the family line, caring for elderly parents and earning money. Many farmers believe that raising a girl baby is a waste because she will marry into another family.

China's problem is compounded by a strict family planning policy that limits many couples to one child. The government credits these restrictions for preventing 400 million births since the rules were introduced in 1979. Without them, it says the world's largest population - now 1.3 billion - would have spiralled out of control. But the result is an increasingly lopsided demography.

In the past, unwanted girls were abandoned. Now they are more likely to be aborted. As China has become wealthier, more couples have access to ultrasound checks. Although there are two laws banning doctors from telling pregnant women the sex of the foetus, the practice is common. Local media report that one common way around the regulations is for doctors silently to give a thumbs-up if the foetus is a boy. If it is a girl, a thumbs-down serves as an execution order.

While Lianyungang had the worst imbalance, it was one of 99 cities with a ratio of more than 125 boys for every 100 girls, says the state-run Xinhua news agency. Citing a report by the China Family Planning Association, it said other provinces with serious problems were Guangdong, Anhui and Hainan. In Guizhou province, the media have reported the existence of 'bachelor villages' where most men of eligible age are unable to find a bride. In cities there has been a rise in commercial matchmaking parties aimed at bachelors, also known as 'guangun', or single sticks.

Family planning experts said the imbalance in China far surpassed the 'normal level', which they defined as an international average of about 105 boys for every 100 girls.

The authorities have found the gender balance harder to manage than restricting population growth itself. In 2003, the government introduced a 'Care for girls' policy, which provided incentives - such as tax breaks and exemptions from school and health fees - for families raising girls.

It also intensified a propaganda campaign in the countryside, where many buildings are daubed with slogans proclaiming 'Girls are as valuable as boys'. Old-style propaganda campaigns are not working, however. This summer, the government said it will punish for the first time any medical institution that tells couples the sex of unborn babies.

It is also trying to standardise its slogan campaign. According to the China Daily, out have gone threats such as 'Your house will be demolished and your cows confiscated if you reject abortion demands', and in have come sensitive entreaties aimed at raising the status of women, such as 'Both boys and girls are the hearts of their parents'.

Whether rural parents agree they are equal remains to be seen.