The government is not doing enough to prevent the trafficking of young people into Britain, a report by two children's charities has said.
Trafficked girls and women are often forced into prostitution
Many are smuggled in from south-eastern Europe to be used in the sex trade, for slavery or for begging, it said.
Unicef UK said ministers needed to do more to help those brought to the UK.
The Home Office said it was committed to combating the "appalling" trade and has made trafficking punishable by up to 14 years in jail.
The United Nations estimates 1.2 million children are trafficked across the world each year - with about 246 million youngsters also thought to be involved in child labour.
In the UK alone, between 1999 and 2003, some 250 children were rescued from trafficking.
But Unicef UK said that figure is "the tip of the iceberg" because there is no coherent system of collating how many people are involved.
In a joint Unicef and Terre des Hommes report called Action to Prevent Child Trafficking in South Eastern Europe (SEE), author Mike Dottridge said the trade could be combated only by addressing the root causes of the problem.
He made the claims after visiting Albania, Moldova, Romania and Kosovo.
He said child protection systems in SEE countries needed to be improved to check that youngsters crossing borders were not being trafficked.
'Scarred and vulnerable'
Youngsters vulnerable to trafficking needed to be given life skills education to teach them how to make decisions and give them greater self-esteem, he added.
Agencies, such as the police, immigration officials and social services should be trained to help them identify trafficked children.
Andrew Radford, deputy executive director of Unicef UK, said: "If a child is picked up who is being trafficked, they might end up in a detention centre with asylum seekers or a foster home.
"These options don't give them a secure place to go. They leave them scarred and vulnerable and at risk of being retrafficked."
Mr Radford said trafficked children needed a "reflection period", where they can be offered counselling, rehabilitation and a chance to decide what they want to do next.
He added: "We need to make sure that children who are rescued are only returned to their country of origin if it's safe to do so and make sure they don't fall into trafficking again.
"The traffickers themselves are very well organised, very flexible and very ruthless, yet the systems that are in place to deal with them are inflexible and unharmonised."
Graham Maxwell is deputy chief constable of South Yorkshire police and programme director at the new UK Human Trafficking Centre which opens in October.
He told BBC Radio 4's Today: "In many cases trafficking is backed up by organised crime networks.
"These networks operate not just across Europe, but are global in their span and it's about making money".
He said it was discovered through Operation Pentameter, which targets sexual exploitation, that an "innocent 15-year-old" could be sold for £8,000.
He said the new centre would look at the welfare arrangements for victims and also aim to increase awareness of the problem of trafficking among people working in the public sector.
Asylum screening units
A Home Office spokesman said Home Office Minister Vernon Coaker was chairing a ministerial group on human trafficking to combat the threat posed by traffickers and to ensure victims are protected.
Ministers were also working with other government departments and groups in the voluntary children's care sector to develop strategies on tackling child trafficking.
"Teams of social workers have already been established at ports, and asylum screening units set up to help identify the particular needs of separated children who may have been trafficked and to help develop plans to safeguard their welfare and protect them from the traffickers," the spokesman said.