'Officers are under stress from the increasing workload and under such circumstances, can our frontline staff check on every child? That is impractical,' he said.

Tai Chi-kwan sparked a 20-hour search after she crossed the border alone last week.

She went missing after being dropped off at a tutorial centre in Yau Ma Tei. She was not seen again until she was found wandering tired and hungry in a Shenzhen shopping centre by three Hongkongers who recognised her from a television report.

Mr Lai said the girl had crossed through the checkpoint when most schoolchildren made their way back to the mainland after school.

'I think our officers handled the incident in an appropriate way.'

Mr Lai said parents should keep their children's travel documents in a safe place to remove the risk of them making unauthorised trips.

But he stressed that officers were trained to check children crossing the border alone, or their parents if they looked suspicious.

Legislator James To Kun-sun said the case was an isolated incident. 'It is very time-consuming to question every child and I think the present system is working quite well,' he said.

But he urged the government to increase the number of frontline staff. 'Officers are under enormous pressure from their increasing workload, as the number of people crossing the border keeps on rising.'

The mother of autistic boy Yu Man-hon, who vanished seven years ago after crossing into the mainland, said security should be tightened at checkpoints and special attention paid to children.

'I understand frontline officers are under stress due to the lack of manpower. But knowing the number of children crossing the border is increasing, shouldn't the immigration chief increase resources for his staff?' Yu Lai Wai-ling said.