Some 20 children in the care of social services in the West Midlands have recently disappeared, probably into the sex trafficking trade, a charity says.
Young girls are often forced into prostitution, the charity said
The children, in Coventry, Birmingham and Solihull, were among 32 young people identified in a recent count as being potential sex or labour slaves.
All the missing children are girls aged about 16, Save the Children said.
The charity is hosting a conference in Birmingham, calling for a multi-agency protocol to tackle the growing menace.
It said the youngsters came to the West Midlands from China, Somalia, Vietnam and Bangladesh, through the people-trafficking route.
Shruti Tanna, regional manager for Save the Children, said that instead of the young people being recognised as needing child protection, they were often processed under immigration and asylum regulations and then vanished - probably back into the sex trade.
"These figures are just the tip of the iceberg. A wide variety of agencies are attending the conference and we must come up with a concerted approach to help these young people.
"The social services are being very positive and we need good practice - led by government - on how to deal with this and how to see these vulnerable young people are protected and cared for in the right way."
Also hosting the conference is Ecpat UK, a group of charities against child exploitation.