The decision proved popular with Hongkongers, who fear the influx of mainland mothers will increase competition for health care and education, but the department says the ban, if extended for the next three decades, will leave the city with 330,000 fewer workers than it had previously expected.

'The decline in workforce, compared to the projection two years ago, is a result of the zero quota and the rising number of elderly [people],' the department's deputy commissioner Leslie Tang said yesterday. 'I do think the government should get prepared for the problem [of an ageing population].'

The previous projection put the workforce in 2039 at 5.47 million, from a population of 8.89 million. The figures released yesterday put the workforce in 2041 at 5.14 million, from a population of 8.47 million.

The city has long had one of the lowest fertility rates in the world. Hong Kong women typically have just one child, while an average of 2.1 children per woman are needed to keep a population steady.

There have been a series of warnings about the risk of an ageing population this year. In February, a government manpower study showed the city faced a labour shortage by 2018, while the government's Steering Committee on Population Policy said in May that the city would have to consider raising the retirement age (there is no statutory retirement age in the city).

More than 40,000 babies were born to mainland women in Hong Kong last year, with mothers-to-be drawn to the city by the high quality of its health care and the prospect of permanent residency for their children.

Tang says previous studies have shown that 98 per cent of mainland women who give birth in the city take their child home shortly after the birth. Some 49 per cent planned to return the child to the city, in most cases before the age of six.

The department expects the annual intake at kindergartens to increase by 20,000 by 2016 as a result of the influx, while the number of primary-school pupils is expected to rise by 85,000 in 2019.

If its projections are accurate, there will be 645 economically inactive people - children and retirees - for every 1,000 workers by 2041, up from a record low of 333 today. If domestic helpers are excluded, the 2041 figure is 712 per thousand, against 352 per thousand today.

The situation would be even worse than it is in Japan now, where there are 565 dependents per 1,000 workers, and also in European countries such as Sweden, Denmark and Britain, where every 1,000 workers support more than 500 dependents.

Figures released by the department last week showed that the city's women now live longer than anyone else in the world, and the projections suggest that the trend towards longer life will continue, with life expectancy for women hitting 90.8 years, from 86.7 last year. For men, the figure would rise from 80.5 years to 84.4.

Previous government projections have been criticised as inaccurate. Tang said the latest ones were not a precise forecast of what would happen and assumed the government would stick with present policies. 'Our approach is internationally recognised,' he said.

Tang said there was still time to deal with the ageing population, as the city's dependency ratio would not return to its 1980s peak for years. But Professor Paul Yip Siu-fai, a population expert at the University of Hong Kong, said the comparison with the 1980s was inaccurate, as most economically inactive people then were children.

'We had more teenagers moving into the workforce in the 1980s. Now we face a situation where many middle-aged people will become elderly. We have no chance of reducing the dependency ratio in this case.'

Yip criticised the ban on mainland mothers. He advocates quotas for educated mainlanders to have children here and join the workforce. He also called for more investment in education to boost productivity.