A population expert has called on China's legislature to make it a criminal offence to identify the sex of an embryo for non-medical purposes and also to outlaw abortions that are not medically justified.

Wei Jinsheng, a researcher with the China Population and Development Research Center, said in an article published in the latest issue of "Qiushi" (Seeking Truth from Facts) magazine that the National People's Congress (NPC) should incorporate provisions banning sex selection for non-medical purposes in the country's criminal law.

The rising gender imbalance is a serious threat to China's smooth development, Wei warned.

In 2004 there were 20 million more Chinese men than women under the age of 20, Wei said.

Many Chinese men in poverty-stricken rural areas cannot get married, leading to social instability, Wei said.

The underlying reason for China's rising gender imbalance is the entrenched idea that boys are better than girls. Ultrasound technology allows prospective parents to learn the sex of an embryo which means that they sometimes abort - particularly in rural areas - when the embryo is female.

Statistics show that 117 boys are born for every 100 girls in China, well above the international average of 104-107 boys, according to Wei.

The government has pledged to bring the gender imbalance under control within five years, but Wei said financial, educational and legal resources are needed to curtail gender selection and abortion for non-medical purposes.

A draft amendment to the criminal law - submitted to the Standing Committee of the NPC for review earlier this year - would institute penalties of up to three years in jail, probation and fines for those involved in gender identification of embryos for non-medical purposes.

But the provision was later removed because lawmakers hold sharply divided opinions on the provision.

Some argue it is necessary to change the country's abnormal sex ratio and prevent abortion being used as a means of sex selection, while others contend that the law is unreasonable and impossible to police as evidence of the proposed crime would be difficult to gather.

Qiushi is a fortnightly theoretical magazine run by the Central Committee of the Communist Party of China.