Scotland Yard today revealed it has been unable to trace all but two of 300 black boys aged four to seven reported missing from school in a three-month period.
Child welfare experts say the number highlights the scale of the trade in children brought to Britain as domestic servants and covers for benefit fraud.
The figure emerged through the murder inquiry following the discovery of a child's torso in the Thames in September 2001. The identity of the victim, named "Adam" by police, is not known but his background was traced to Nigeria, it is believed he died in a ritual sacrifice. Detectives asked each London local education authority to give them details of black boys aged four to seven reported missing from school between July and September 2001. It emerged 300 had vanished, 299 from Africa and one from the Caribbean.
Police only managed to trace two children. The true figure for missing boys and girls is feared to be several thousand a year.
Detective Chief Inspector Will O'Reilly, leading the hunt for Adam's killers, said: "It is a large figure, far more than we anticipated. 300 young boys didn't return to school and are really lost in the system."
Police visited the children's addresses, but in most cases were told the boys had returned to Africa. Inquiries undertaken via Interpol in the boys' home countries failed to trace them.
Journalist Yinka Sunmonu, an expert in the issue, told the BBC's Today programme: "Children are being trafficked. There is domestic slavery, physical abuse, sexual abuse. Children are ... here one day and gone the next."
The Met's recent Paladin child investigation found hundreds of unaccompanied children arrive in Britain each month.