Jon Fayle, who until his resignation in November was officially responsible for cutting the number of children in custody, blamed the political climate for the number held in England and Wales.
His decision to speak publicly about the state of the youth justice system follows the resignation two weeks ago of Rod Morgan as chair of the YJB. He says Professor Morgan fought an uphill battle over the last three years against unwilling ministers to reduce the use of child custody.
"The level of custody for children in this country is little short of a national scandal, and the principled resignation of Prof Morgan has thrown the issue into sharp relief," Mr Fayle writes.
He says that the YJB, which commissions custody for children, spends ?280m a year - 70% of its budget - on locking up just 4% of young offenders, of whom 78% reoffend within 12 months of their release. "Leaving to one side for the moment the powerful arguments about children's rights, and the humane treatment of children, this cost-benefit analysis is damning in the extreme."
He says that the YJB drive to reduce custody cut numbers between 2002 and 2006 by 3% to 2,995, but they have since risen. He blames the "political climate set by ministerial statements and attitudes.
"The most important factor is political. The political mood music playing in the minds of sentencers as they make their decisions is, I would argue, the critical factor," says Mr Fayle, blaming the competition between the main political parties to be tough on youth crime for the doubling of youth custody at a time when juvenile crime was falling.
He also says the way the budgets work is a factor promoting the high use of custody. The YJB has a central budget to pay for secure accommodation and when a child from a particular local authority is sent to custody it is, generally speaking, a free service. "A child who may have been costing the local authority a lot of money in terms of an expensive placement programme, and copious staff time, is suddenly off their hands - and at no cost."
If the local authority had to foot the bill it would be a strong financial motivation for them to avoid sending children to custody, he argues.
Mr Fayle calls for a reduction in the use of child custody, saying that the release of funding would make it possible to improve services for those who genuinely need to be locked up to protect the public, and to provide more preventive services for younger children at risk of crime.
He calls for ministers to give explicit support for the objective of reducing the use of youth custody and all-party recognition of the futility of high custody levels for children. He argues a "custody threshold" should be enshrined in law so only those who are a danger to the public or who have persistently breached community punishments should be locked up.
Graham Robb has been appointed the YJB's interim chairman while a replacement is found. It is believed that Louise Casey, the "respect tsar", is being backed by Downing Street for the job.