Council leaders are calling for urgent action to ensure children and teenagers held in youth custody are safe.
The Local Government Association wants the Government to publish a clear action plan to improve conditions in young offender institutions and secure training centres.
Earlier this year the prisons watchdog described the speed of decline in the custodial estate for children and young people as "staggering".
Concerns have been raised over levels of self-harm and violence in the establishments.
Richard Watts, chairman of the LGA's children and young people board, said: "Councils take their responsibility toward child safety extremely seriously, and work hard to ensure that children and young people are never put in situations that would put them at risk.
"There is no other situation in which children and young people would be placed into environments that are known to be unsafe, and youth custody should be no exception."
He added: "Her Majesty's Prison and Probation Service were made aware of these issues in July, yet we still have no clear idea of what action will be taken, and by when, to rectify the situation and make sure our young people are safe in custody.
"This situation would not be acceptable for local authorities, schools or any other public institution charged with the care of children, and it should not be acceptable for HMPPS.
"Action needs to be taken to ensure that young people are safe in custody."
A Ministry of Justice spokeswoman said: "The safety and welfare of every young person in custody is our absolute priority and we are clear that more needs to be done to achieve this.
"But we also want custody to improve the life chances of children in our care and to deliver improvements to education and health services within youth custody.
"That's why we have created a new Youth Custody Service, with an executive director for the first time in the department's history - to make sure this vital area is given the priority and weight it deserves.
"The new director will lead the implementation of reforms to the running of the youth estate, including boosting the number of frontline staff by 20% - all of whom will be specially trained to work in the youth estate."
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