A report by the Prisons Inspectorate found that levels of violence and the use of force at Feltham A, which held 126 15- to 18-year-olds at the time of the inspection, had increased since the last inspection in August 2015, with some of the violence deemed to be "very serious", including multiple assailants and the use of weapons.
Inspectors said the response in terms of behaviour management was "ineffective", with a focus on sanctions and regime restrictions. Meanwhile, time out of cell was "inadequate" and boys were prevented from using basic amenities, including showers and telephones.
The restricted regime that was in place meant 40 per cent of the boys were locked up during the school day, while 30 per cent were out of their cells for just two hours each day.
Inspectors also found that despite sufficient school places and teachers, fewer than half the boys were getting to classes.
Youth justice campaigners have called for urgent action on the findings.
Carolyne Willow, director of children's rights charity Article 39, said the report is evidence of "systemic child cruelty and neglect" and shows that "children are unsafe". "This is a child protection scandal that demands emergency action to safeguard children's welfare," she said.
"Feltham's child prison must be closed, children moved to safe environments and then an independent review commissioned about the transfer of responsibility for children in custody to a government department which has the knowledge, capacity and value base to look after children well."
Alexandra Wigzell, chair of the Standing Committee for Youth Justice, said the report shows how badly children in custody in England and Wales are now being looked after.
"This is a system in deep and growing crisis," she said, adding that the rise in violence, the use of unsanctioned control and restraint techniques on children, and the increasing habit of locking children away in their cells for long periods of time are evidence of a "national scandal".
"This requires urgent and immediate action from the government," she said.
"I am calling today for the government to publish, immediately, its timetable for the decommissioning of YOIs and STCs, as well as the implementation of its other reforms to increase resources in YOIs."
Frances Crook, chief executive of The Howard League for Penal Reform, said: "These children are suffering abuse and neglect by the state.
"Feltham has failed to care for children and help them turn their lives around for decades. It is time to put an end to this abusive failing system and properly help children live law-abiding lives."
However, inspectors did find a number of positives. The report reveals that despite the fact circumstances for staff were "very challenging", most interactions between staff and boys were polite.
Meanwhile, care for boys in crisis or at risk of self-harm was "reasonably good", substance misuse services remained good, health care was good, and the work of the mental health team was "excellent".
And despite problems caused by the regime, the education provider had created a positive school ethos with high expectations.
Peter Clarke, chief inspector of prisons, said violence was a "serious problem" at Feltham A, adding that during the inspection there was a serious assault on an officer.
"Staff should be able to work in a safe environment and not be in constant fear of being assaulted," he said.
"The current approach is failing to deliver that reasonable expectation and from the evidence available to us, is actually making it worse. The focus on keeping people apart rather than trying to change their behaviour has not worked. Feltham A is not safe for either staff or boys."