By Matthew Chapman
BBC Radio Five Live
There are fears that large numbers of children may be trafficked into Britain after police discovered up to 30 had been "lost".
Many of children trafficked to the UK are believed to be Nigerian
The BBC has learnt the Metropolitan Police investigation looked at children who came through Heathrow Airport with adults who were not their parents.
Until recently few checks have been made by immigration at the airport.
Campaigners fear thousands of children are being used as domestic slaves after being brought into Britain.
Children often arrive at Heathrow accompanied not by parents but by adults who claim to be uncles or aunts.
Sometimes the children arrive alone at the airport and go through immigration to be met by adults who claim to be relatives.
The Metropolitan Police launched Operation Palladin Child last September and for the next three months they went and knocked on the doors of the addresses given by the children when they passed through immigration.
The police focused only on a sample of addresses which they thought were suspicious.
Neither the Metropolitan Police or the Immigration Service would officially release the findings but BBC Radio Five Live understands the children could not be found at up to 30 of the addresses visited during that three month period.
Police discovered that in some cases the same address had been given by successive children who could not be traced.
Some of the houses they visited were so called "hot" addresses which means they were already associated with criminals.
Several addresses were well known to officers who work on Operation Trident which targets gun crime in the black community.
'Aunts' and 'uncles'
The vast majority of the children were from Nigeria, which is well known among law enforcement officials as being the main source for trafficked children into Britain.
"For years these children have been arriving at Heathrow and we've been sending them through without having a clue where they were going to," said a source familiar with the investigation, who declined to be named.
"Sometimes the children would be all lined up at Heathrow with bits of paper hanging from around their necks, these were letters supposedly from their parents saying it was all right to pass them through immigration to their so called uncles at the other side.
Police made checks on children coming through Heathrow Airport
"We've been doing this for years without knowing what's happening to these kids," he said.
The police operation found that up to 190 unaccompanied children pass through Heathrow Airport every week.
Campaigners say child slavery is rife in Britain.
Debbie Ariyo of Africans Against Child Abuse said: "They are used mainly as free child care.
"They are brought into this country to look after the children of African couples living in this country.
"Back home it is the culture to use children for domestic work. But here they don't go to school, they have to work all day and they are then at risk from abuse."
In statements the Immigration Service said the government acknowledged that trafficking was a terrible crime and was determined to tackle it.
The Metropolitan Police said: "It would be unhelpful to discuss the data collected during the scoping study until it has been fully analysed."
Matthew Chapman's documentary The Five Live Report: Please Look After This Child was broadcast at 1100 GMT on Sunday and repeated at 1930 GMT on Radio Five Live.