From this month, 4.1 million non-permanent residents of Shenzhen will be allowed to apply in the border city for multiple-visit permits to Hong Kong. Previously they had to apply in their home provinces.

Ngai Sik-shui, chairman of the Immigration Service Officers Association, said pressure on staff at border checkpoints was mounting daily as the city struggled to cope with the increasing number of mainland travellers entering Hong Kong.

Tackling pregnant mainland women without a registration certificate from the city's hospitals and taking care of cross-border schoolchildren had also increased their workload, he said.

"The increase in manpower cannot be delayed. It's at a critical point now," he said. He urged the authorities to cut through red tape to speed up procedures.

Ngai did not estimate how many more visitors officers would handle after the relaxation of entry permit rules, but said staff now had to work two hours non-stop, without toilet breaks, and the changes would exacerbate the situation.

The unions identified the Lo Wu, Lok Ma Chau and Western Corridor border crossings as bottlenecks.

Alex Chu Wai-ting, chairman of the Rank and File General Union, said its members had demanded increased staffing for years, but government bureaucracy had made the process slow.

"When we heard about the relaxation, we were shocked. We do not know how to handle such an increase," he said.

Chu said many staff were working 12 hours a day and coping with the volume of visitors by working overtime.

The Immigration Department reported had 5,321 uniformed staff at the start of last month. It pledged to deploy resources according to the number of visitors.

According to figures from the Tourism Board, more than 3.2 million mainland visitors came to the city in July.