It is the second successive month that the figure has risen - bringing the total to the highest level since December 2015, when there were also 927 under-18s in custody.
Jacob Tas, chief executive of social justice charity Nacro, which is developing plans to improve the way young offenders are supported on leaving custody alongside the YJB and Ministry of Justice, said: "It is very disappointing that figures from the YJB show that youth custody levels are showing a continuous rise for the first time in almost a decade, with over six per cent more children and young people aged up to 18 within the secure estate, in comparison to 2016."
"With the juvenile proven reoffending rate following custody currently standing at 69.4 per cent, it is now more likely that increasing numbers of young people will get caught up in a destructive cycle of offending that they will struggle to get out of."
Tas said there is overwhelming evidence that the majority of children and young people in custody come from disadvantaged families and communities, with experiences of childhood trauma such as neglect and abuse.
"Failing to identify trauma and mental health concerns at the earliest opportunity means pre-existing trauma and mental health concerns will get worse during time in custody," he said.
"This leads to the type of dysfunctional behaviour that causes repeat offending.
"To ensure that this worrying trend does not continue, there must be a relentless focus upon putting resettlement at the heart of a custodial sentence, so that young people can take the important steps they need to move on from crime, identify and achieve their goals, and flourish in our communities."
Youth custody levels fell to a historic low of 829 in December 2016, but have been rising since then.
The YJB, which was relieved of responsibility for youth custody in April, has said it is monitoring the situation.