Runaway children as young as nine are being forced into prostitution, drugs and crime, according to a report.
Around 100,000 under-16s run away from home or care each year
The Children's Society said that rather than sleep rough, many youngsters are "befriended" by adults who sexually abuse them or introduce them to drugs.
The charity has urged the government to set up a national network of safe emergency accommodation for under-16s.
It is thought that around 100,000 youngsters aged under 16 run away from home or local authority care each year.
Runaways effectively "disappear off the radar" of the social services and other officials, warned the charity.
Charity chief executive Bob Reitemeier said: "No child should have to resort to prostitution, drugs and crime to survive in Britain today.
"We need to act now to ensure they have somewhere safe to stay, out of harm's way."
Disturbing cases uncovered in the charity's report include a 13-year-old girl runaway who was abducted by a drug dealer and held prisoner in a flat for four months.
It also reports on a 15-year-old girl who was forced into prostitution by an older boyfriend to fund his heroin addiction, and a 14-year-old boy who was offered a place to stay by a friendly couple who then locked him in a cupboard.
The teenager attempted suicide by slashing his wrists and drinking bleach.
The charity's research involved a series of 23 interviews in six areas of the UK last year with youngsters aged 13 to 21 who had run away from home, been thrown out or abandoned by their parents before the age of 16.
Previous work by the charity revealed that a quarter of first-time runaways were under 11, and a quarter ended up sleeping rough.
The findings also showed that one in seven young runaways were physically or sexually assaulted while away from home.