Some children climb out of windows or escape during fire drills, authorities say
Gordon Brown has vowed to investigate reports that criminal trafficking gangs have been targeting a children's home next to Heathrow airport.
More than 80 Chinese children have gone missing from the home since 2006, a report from the UK Border Agency seen by the Guardian newspaper states.
Hillingdon Council said it was unable to stop the children from leaving as it would breach their human rights.
The prime minister said the government was committed to stopping trafficking.
The report, which was marked "restricted", said the centre had become a "clearing house" for international gangs.
It says Chinese children who arrive alone at the airport are taken into local authority care, and in two thirds of cases vanish within a week.
Some simply walked out of the building to be picked up by waiting cars, while others escaped out of windows or during fire drills, it said.
In one year 77 children fled the centre and only four were found. Two girls, one of them pregnant, later reappeared after being forced to work in brothels in the Midlands.
Others are thought to be coerced into becoming street sellers or working in the drugs trade.
Julian Wooster, Hillingdon Council's deputy director of Children and Families said: "We cannot lock the doors because it's a breach of their human rights.
To have such a large number of children going missing when they are supposed to be in care is unacceptable
Shadow Home Secretary
"Unless they have committed a crime we do not place them in a secure setting. We try to persuade them not to run away from the centre."
The leaked report says: "The absconding may be at the facilitation of organised crime groups and the children may then be exploited for financial gain."
Shadow home secretary Chris Grayling said the report highlighted a "scandalous situation in our immigration system".
He said: "To have such a large number of children going missing when they are supposed to be in care is unacceptable. We need an urgent explanation from the home secretary."
When asked about the report at Prime Minister's Questions, Mr Brown said: "Child trafficking is completely unacceptable and inhumane. Anything that we can do to stop child trafficking, we will.
"We will do everything we can to protect these children and we are leading internationally in asking other countries to help us ban the practice of trafficking children," he added.
A spokesman for the UK Border Agency said: "We are having real success targeting the routes used by the criminals who prey on these vulnerable youngsters - a joint operation has seen 12 traffickers prosecuted in the past 12 months alone."
Local authorities have a duty to ensure children are kept safe from harm, he added.
A Department for Children, Schools and Families spokeswoman said it was clearly "very concerned" about any children that went missing from local authority care.
"It is a duty for local authorities to make sure looked-after children are properly safeguarded wherever they are placed.
"They are also responsible for making sure the necessary measures are in place to take action whenever a child in their care goes missing."