Hundreds of African boys have disappeared from London schools, police investigating the murder of a boy whose torso was left in the Thames have said. Scotland Yard asked London education authorities how many black boys aged four to seven had vanished from school.
Between July and September 2001, 300 children had disappeared and police fear thousands may go missing annually.
Child welfare experts say the figures show the scale of child trafficking, sometimes for labour or benefit fraud.
A previous investigation by BBC Radio Four found that some African children were being held by their parents' creditors, so they could claim extra benefits.
'Lost in the system'
The boy found in the River Thames in September 2001 - called Adam by police - is thought to have been the victim of a ritual killing after being brought to London from Nigeria.
Detective Chief Inspector Will O'Reilly said inquiries revealed 300 black boys of a similar age to Adam had not reported back to school in the three months before his death.
It is a large figure - far more than we anticipated when we started this line of inquiry," Mr O'Reilly told BBC Radio Four's Today programme.
"From the June to the September, 300 young boys between four and seven - and only between those age groups - didn't return to school and are really lost into the system."
Of the missing boys, 299 came from Africa and one from the Caribbean.
Despite an international search, police were able to find only two of them, Mr O'Reilly said. Most of those questioned said the children had returned to Africa.
While there is nothing to suggest these children have been murdered, a lack of immigration records makes them almost impossible to trace, police say.
Author and journalist Yinka Sunmonu said more should be done to protect young people brought to the UK.
"They are being trafficked, they are being emotionally abused, there are incidences of domestic slavery, there is physical abuse, sexual abuse," she said.
But Ms Sunmonu acknowledged it was hard to track a "secretive" trade where children were often passed from one adult to another.
The BBC's Angus Stickler says part of the problem lies with the custom in many African communities of placing children with distant relatives.
The government does not currently force adults who take children in these circumstances, known as private fostering, to register. It fears doing so could drive the problem underground.
Felicity Collier, chief executive of the British Association for Adoption and Fostering, said the problem was already underground and she called for tighter controls.
"We would not accept this as a society if these were white children - we have to have a law in this country that says that private foster carers have to register," she said.
Under the 2004 Children's Act, local authorities are meant to advise people they must notify social services if they are caring for a child.
Police and welfare groups hope reforms - being implemented this year - will make it easier to track vulnerable children.
But the shadow secretary of state for children said the Children's Act needed to go further, with a system allowing for prosecution of those who failed to register as carers.
Conservative Theresa May told BBC News: "At the moment the onus is on the local authority to try and find them, to search them out. We think that the onus should be the other way around."