Children and young people are being failed by the systems set up to safeguard them from harm, the report by the Prison Reform Trust and the Inquest campaign group found.
Nine children and more than 190 other young people aged 24 and under have died in prisons or secure training centres in the last ten years, campaigners have said.
The report, which looked at the experiences of 98 children and young people who died, found they were "some of the most disadvantaged in society" and had experienced problems with mental health, self-harm, alcohol and drugs.
In many cases there were communication failures between community agencies and prisons while, in others, they were placed in prisons with unsafe environments and cells, the report said.
It added that poor medical care and limited access to therapeutic services in prison also caused problems and some children and young people had been exposed to bullying, segregation or restraint.
More top news
The Duke of Cambridge has condemned the "violent and apparently targeted murder" of the head of a wildlife conservation foundation.
Experts are warning that millions of middle-aged people are putting their health at risk due to a severe lack of exercise.
Teenagers across Wales, Northern Ireland and England are to get their results, with much of the focus on the effect of reforms in England.