Almost half of boys held in youth offender institutions are from a black or minority ethnic background, surveys conducted by a watchdog indicate.
The disclosure emerged in a report released by HM Inspectorate of Prisons, which has published an annual summary of responses from those in YOIs since 2001/2002.
Findings in 2015/16 showed the proportion of boys who said they were from a black or minority ethnic background was at 47%.
This was the highest rate recorded since 2001, the report said.
It added that those with experience of the local authority care system (37%), Muslim boys (22%), boys reporting a disability (19%) and those identifying as being from a Gypsy, Romany or Traveller background (7%) "continued to be disproportionately over-represented" across the YOI estate when compared with the population as a whole.
Survey findings for secure training centres, another type of custodial facility for children, showed the proportion who identified as being from a black or minority ethnic background was 41%.
The number of children in custody fell by 53% between 2010-11 and 2015-16. As of April, there were 906 under-18s being held in England and Wales.
The HMIP report, commissioned by the Youth Justice Board, analyses 12 to 18-year-olds' perceptions of their experience in custody.
It found that nearly half of boys (46%) in YOIs reported feeling unsafe in custody at some point - the highest figure recorded through the surveys.
The proportion of boys with a job in their establishment had fallen significantly in the past 12 months to 16%, compared with 28% in 2014-15, the study added.
Chief inspector of prisons Peter Clarke said: "Over the past decade the number of children in custody has fallen by some 66%, but the perceptions of those that remain leave us with some worrying and difficult issues to consider.
"There are some particularly troubling findings in the areas of disproportionate over-representation (in terms of the characteristics of the children now being held in custody), safety, victimisation, respect and training.
"I hope these findings are taken seriously by those charged with developing and improving policy."
Colin Allars, chief executive of the Youth Justice Board, said: "Parts of this report are uncomfortable to read - trends around safety are concerning.
"We will use these findings to support our work with providers of custodial services to address the issues children and young people are telling us about."
Frances Crook, chief executive of the Howard League for Penal Reform, said: "It is shameful that the Government spends millions on locking up children yet they are unsafe inside and lack support before and after custody. The prisons are a disgrace and should be closed.
"Part of the problem is that prisons are the end of a torrent and it is the courts that must look to their disproportionate use of custody for BAME (Black and minority ethnic) children."
A Ministry of Justice spokeswoman said: "These findings are concerning.
"The welfare of young people in custody is extremely important and the Justice Secretary has been very clear that we need to do more to improve safety and help offenders turn their lives around.
"We will be setting out our plans to reform the way we manage young offenders in the coming weeks."