The southern Chinese boomtown of Shenzhen announced last week that it would allow an extra 4.1 million of its residents to obtain multiple-entry permits for Hong Kong, a former British colony that reverted to Chinese rule in 1997.

But just a day before the scheme was to take effect, Hong Kong's new leader, Leung Chun-ying, announced it would be put on hold for three weeks after concern was raised with Beijing that the influx could strain the city's clogged border checkpoints, tourism spots and teeming streets.

Last year, there were 28.1 million mainland Chinese tourists to Hong Kong, almost four times the city's population, up from 8.4 million in 2003. In the first half of this year, 15 million mainlanders arrived, a 23 percent jump over the previous year.

"Many people, including myself, are paying heed to Hong Kong's capacity and ability to absorb," said Leung, as he announced that Shenzhen authorities would not issue any new visitor permits for three weeks.

"We will engage the central government and other authorities on the mainland for us to mutually discuss the capacity of Hong Kong to receive these additional visitors."

Leung skirted questions about whether he would push Beijing's leaders to restrict the numbers of mainland visitors allowed into Hong Kong.

For decades Hong Kong was a racy capitalist outpost on communist China's coast. Even though Chinese cities are now growing fast, cosmopolitan Hong Kong is a favourite destination for people from the mainland.

The freeing up of Hong Kong's border to mainland visitors since 2003 has been a major boon to the tourism, retail and catering sectors. But resentment has been building.

Hong Kong people complain that the influx has driven up property prices and other costs, posing a problem for Leung whose popularity ratings have plunged over a housing scandal and various contentious policies.

Pregnant mainland women have crowded into Hong Kong's maternity wards, eager for their children to be get the right to live in the city.

Even tourism officials now say the volume of Chinese visitors may be reaching unsustainable levels. Social groups, immigration officers and politicians campaigning for city elections have called for a rethink of Hong Kong's integration with China.

Ngai Sik-shui of the Hong Kong Immigration Officers Association said an extra 400 staff would be needed if the plan to allow more visitors went ahead.

"At every checkpoint, we have experienced stress and pressure," Ngai said. "This pressure is building every day." (Additional reporting by Tara Joseph, Sisi Tang and Tan Ee Lyn; Editing by Robert Birsel)

• A Permanent Economic Emergency