The boy's mother, Yu Lai Wai-ling, said the mistake was 'irreparable', and feared the worst. The Security Bureau said the Government was deeply concerned and an internal investigation had been ordered.

Man-hon ran away from his mother in Yau Ma Tei MTR station at 11am on Thursday. He was seen by mainland officers at the Shenzhen border later the same day. They asked the SAR side to confirm the boy's identity.

An SAR immigration officer interviewed the boy between 5pm and 7pm. But a department spokesman said that because the boy did not respond to questions and had no documents and because there was no information to show he came from the SAR, he was handed over to Shenzhen at 7pm - handcuffed, to avoid injuring himself or others.

The spokesman said the officer did not suspect the boy - who was highly emotional, urinated and spat in the interview room and threw food and water at the officer - had a mental disability. 'There was no justified reason for us to call for professional assistance,' the spokesman said.

The acting principal immigration officer at Lowu, Tam Wing-yin, could not explain how the boy passed through the border unnoticed. 'This is a very rare case. We have already started an internal probe into the incident,' he said.

Mrs Yu, who lives in Lok Fu Estate, Wong Tai Sin, said: 'It's unacceptable and an irreparable mistake that the officers didn't bother to check his identity carefully before sending him off as an alleged illegal immigrant.

'He's obviously mentally disabled and unable to express himself. We're all very worried and everyone has been running around for the past few days. I haven't eaten since last Thursday. I'm heartbroken that my son is suffering 10 times more than me out there. He can't take care of himself.'

The Hong Kong Regional Missing Persons Unit dispatched a search team to Shenzhen yesterday with Mr Yu and has sought co-operation from the Shenzhen Public Security Bureau through the Hong Kong Police Liaison Bureau.

'What happened is terrible - this is the most serious mistake I have ever heard of,' the chairman of the Human Rights Commission, Ho Hei-wah, said. 'The Immigration Department has not treated the boy as a human being. Any person with minimal conscience would not have abandoned a child in Shenzhen, especially if he cannot express himself.'

Mr Ho said the department should have given the boy the benefit of the doubt, sent him to a children's home and then checked his identity.

But Democratic Party member and former legislator James To Kun-sun said the handling of the case was acceptable. 'The boy was found in Shenzhen. The burden of proof of the boy's identity should be on the Shenzhen side.'