Children from gypsy, traveller or Romany backgrounds make up a "hugely disproportionate" number of inmates in youth prisons, figures reveal.
One in 10 (11%) of those held in secure training centres (STCs) said they were from these groups, despite making up an estimated 0.1% of the overall population.
In addition, one in 12 detainees (8%) at youth offender institutes (YOIs) considered themselves to be gypsy, Romany or traveller.
The findings emerged in an annual summary of surveys conducted in the two main types of facilities for juvenile offenders in England and Wales by HM Inspectorate of Prisons.
Chief inspector Nick Hardwick said: "We have repeatedly raised our concerns about the hugely disproportionate number of children in custody from a Traveller or Gypsy background.
"With any other group, such huge disproportionality would have led to more formal inquiry and investigation."
The proportion of boys held in young offender institutes who are from ethnic minority groups has nearly doubled in 13 years from just under a quarter (23%) in 2001/02 to 42% in 2014/15, although this was down from 45% in the previous year.
The number of Muslim boys in YOIs has risen from 16% in 2010-11 to 21% in 2014-15. There were differences in the demographics of STCs, with falls observed among those who were from BME and Muslim backgrounds.
There have been sharp rises in the share of children who consider themselves to have a disability or who have been in local authority care in the last five years.
Overall, the number of children in custody fell by 49% between 2010-11 and 2014-15.
The report said there is some evidence to support the suggestion that as the number of boys in custody has fallen, those who remain are "a more concentrated mix with more challenging behaviour and complex needs".
The report also found that:
:: Although around four in five children described feeling safe on their first night, almost a third said they had felt unsafe at some time.
:: Two out of five children said they had been physically restrained.
:: Only slightly more than half the children felt they had done anything in custody to make them less likely to offend in future.
:: Fewer boys in YOIs reported being involved in any kind of purposeful activity than at the time of any inspection reports in the past five years.
Lin Hinnigan, chief executive of the Youth Justice Board (YJB), said: "We are concerned at the number of young people who do not feel safe at some point during their time in custody.
"The purpose of custody is to help rehabilitate young people so that they can live crime-free lives, so reduced levels of purposeful activity are also troubling.
"We are working closely with those running secure training centres and YOIs to ensure they improve and meet the needs of each individual child in their care.
"The YJB is also committed to working with partners to address the issue of minority groups - including gypsy, Romany and traveller - being over-represented in the youth justice system."