Demographers doubt the campaign will do much to reverse a trend which has seen the number of retirees grow more than twice as fast as the general population since 2000, leaving the city with 21.6 per cent of its people aged over 60 and only 10.6 per cent under 18.

There was little drive to have children in modern society, said one expert, Peng Xizhe , director of Fudan University's Institute of Population Research.

Officials are in no doubt as to the seriousness of the problem. 'The rising numbers of retirees will put pressure on the younger generation and the social security system,' Xie Lingli , director of the Shanghai Population and Family Planning Commission, told Xinhua.

Couples are allowed a second child if both are only children or have doctorates, if they are disabled, if they are rural and their first child is a girl or if a divorcee marries someone childless.

More couples have had a second child since 2004, when Shanghai scrapped a requirement that there be a gap of at least four years between the births of first and second children. Still, Professor Peng said some of the people born under the one-child policy do not want to get married or have children.

Zhou Haiwang , deputy head of population and development studies at the Shanghai Academy of Social Sciences, sees another problem. Public opinion needs moulding to 'let people believe having a second child is not burdening society'.

He advocates incentives such as kindergarten subsidies or extra years of free education for second children.