Statistics published by the Ministry of Justice show that as of April, there were 940 under-18s in youth custody, of which 452 (48.1 per cent) were BAME, while the ethnicity of a further 18 young people (1.9 per cent) was classed as "unknown".
The remaining 470 young people in youth custody - exactly 50 per cent of the total - were classed as "white".
The figures represent a significant increase in the proportion of BAME children in custody since the start of the year. In January the proportion stood at 42.6 per cent.
The figure has been consistently rising over the past decade. In June 2008, the proportion of young people who were black, Asian or from an ethnic minority background stood at 25.3 per cent, compared with 67.1 per cent who were white, with the ethnicity of 7.6 per cent not known.
Across the UK population as a whole, 87 per cent of people are white, and 13 per cent belong to a black, Asian, mixed or other ethnic group.
The increase comes despite acknowledgement of the issue within government and efforts to address it.
The Ministry of Justice is currently exploring the idea of deferring prosecutions, offering young people the opportunity to undertake an intervention, such as drug treatment, prior to the point of criminal prosecution.
Meanwhile, under new guidance that came into force last summer, courts will be required to take into account the over-representation of black and ethnic minority children in the youth justice system when they sentence children.
And in 2016 the Youth Justice Board launched a bid to get more students from BAME backgrounds to seek a career in youth justice.
Frances Crook, chief executive of the Howard League for Penal Reform described the increasing proportion of BAME young people in youth custody as "an absolute disgrace".
"I call on government and the Mayor of London to respond to David Lammy's review on the treatment of, and outcomes for black, Asian and ethnic minority individuals in the criminal justice system pretty damn quickly.
"There also needs to be leadership from the minister responsible for children in custody with regards to the behaviour of all those white magistrates."
A Ministry of Justice report published in November 2016 on BAME disproportionality in the criminal justice system in England and Wales found that during 2014 BAME boys aged 10 to 17 were 35 per cent more likely to be arrested than young people of white ethnicity.
The Ministry of Justice has been contacted for comment.