'Couples, particularly the younger ones, should at least give birth to three kids to help alleviate the ageing population,' Mr Tsang said yesterday on a radio programme.

Financial Secretary Henry Tang Ying-yen responded last night that he would consider increasing the tax allowance for children.

Hong Kong's birth rate was already the lowest in the world at 0.94 per woman of child-bearing age in 2003. It is understood that last year's rate, to be announced in the next two months, was about 0.8.

If the government introduces incentives to reverse the trend, it will be following in the footsteps of Singapore, where the birth rate in 2003 was a record low 1.26. Singapore offers parents cash gifts ranging from S$3,000 to S$6,000 ($14,287 to $28,574) for the first to fourth child and matches dollar for dollar parents' savings in special accounts for their children.

On present trends, by 2031 a quarter of Hong Kong's population will be 65 or over. The gap is not being closed by the usual supply of mainland migrants, as last year's arrivals were down 36 per cent to 38,072, from 53,507 a year before.

The comments by Mr Tsang, who heads the Taskforce on Population Policy, came after he was asked on RTHK's Talkabout programme yesterday whether raising the tax allowance for third children and beyond could be considered.

Mr Tsang said men going north to marry were not helping things, as there would be just 70 men for every 100 women by 2031.

Responding to a tongue-in-cheek question why many men still opted for marriage across the border given the slimming efforts of many Hong Kong women, he said: 'Perhaps the gentlemen in Hong Kong are not handsome enough.'

Mr Tsang also said the city needed to attract more talent by relaxing rules on the admission of mainlanders. But he stressed the daily 150 one-way permit quota should not be used for any purpose other than family reunion even if the quota was not met.

Paul Yip Siu-fai, senior lecturer with the University of Hong Kong's department of statistics and actuarial science, said it would be difficult to turn the tide of late marriages and the increasing number of women staying single.

The number of single women had overtaken married ones by 70,000, he said.

'Our research shows that people who are already married have an average of two children. It will be very difficult to ask the people who have two to go for three,' he said.

'It is much easier to ask those who do not have babies to have one or two. So the emphasis should not be so much on married couples, but rather on the never-married.'

Dr Yip said the government should start a targeted population campaign - providing an environment conducive to marriage and starting a family.

Lui Tai-lok, a professor with Chinese University's department of sociology, said tax incentives alone could not persuade couples to have more children.

'The decision to have kids depends on a host of factors, like the spouses' evaluation of the education system and prospects of Hong Kong society,' he said.

The Central Policy Unit is conducting a study on how Hong Kong could learn from other countries' experience in attracting quality migrants and encouraging childbirth. The study is due to be completed by the end of the year.