Campaigners have called for urgent action to stop unaccompanied child asylum seekers being thrown behind bars with adults.
Traumatised youngsters whose ages are disputed are being imprisoned on the basis of guesswork by officials, according to the Refugee Council.
The Government pledged to end child detention in 2010 but children are still slipping through the net because there are too few safeguards, the charity warned.
It has secured the release of 17 young people wrongly classed as adults since January and 120 over the previous four years, although it claims the scale of the problem is likely to be far greater.
The council called for immediate reforms that would mean every young person who claims to be a child is given a swift age assessment by experienced social workers.
Judith Dennis, policy manager at the charity, said: "Unaccompanied children are among the most vulnerable children in Britain. They have often witnessed horrors most grown-ups would struggle to imagine and they arrive here all alone and extremely traumatised.
"These are children who desperately need our protection, yet their safety is being unacceptably jeopardised by the authorities. It's clear that the stakes are far, far too high for children to be arbitrarily thrown behind bars with adults on the basis of guesswork.
"The Government must immediately take steps to ensure that its immigration policies don't put children at further risk: children's safety must always come first."
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