Since the late 1970s China has pursued a one child policy, but rural couples with a girl or members of ethnic minorities have had the right to a second child.

However, migrants who work in cities without official residency papers (and without access to social services like health care and public school) have been able to get around the law and have more than one child.

According to official figures, there are about 150 million migrants, 70 per cent who can are of child-bearing age.

The new rule was approved on 29 April and is set to come into effect on 1 October.

Migrant workers who comply with the new rule would be entitled to a string of new benefits including, including a paid vacation if they spontaneously undergo sterilisation and free contraceptives. 

For China’s authorities controls are essential to check population growth. But the effect of the policy has been to distort the country’s demographic profile and favour selective female abortions because of traditional preference for boys.

Despite the crackdown dissatisfaction has been growing. In January a survey commissioned by the National Population and Family Planning Commission showed that 70.7 per cent of Chinese women would like to have two or more children.

At present some 90 million people have no siblings and sooner or later a small number of young adults will have to bear the burden of a very large older age cohort.