A prison officer was assaulted during the inspection of a jail where violence was found to be a serious problem.
HM Prison and Young Offender Institution (YOI) Feltham was judged to be unsafe, with inspectors expressing disappointment in the facility's standards.
Young teenagers ate all meals alone, locked in their cells, a report carried out earlier this year said.
The prison in Middlesex, which is split into two, was holding just over 500 people at the time of the inspection.
The report found some serious violence involving multiple people and weapons in Feltham A, which caters for boys aged between 15 and 18.
Unpredictable and reckless violence included groups of boys kicking and punching each other, inspectors said.
Inmates were locked up for almost 20 hours a day on average, and the focus on sanctions contributed to a cycle of violence, the report added.
But inspectors said the environment posed a challenge for staff, who they said mostly remained supportive and sensitive to the boys in their care.
Following the serious assault on an officer, staff remained professional and engaged with the boys, HM Chief Inspector of Prisons Peter Clarke said.
Efforts to keep people apart rather than addressing their poor behaviour has made the prison unsafe for both the staff and the boys they care for, he added.
There was found to have been a significant increase in violence in Feltham B, which holds young adult men aged between 18 and 21, since its last inspection in July 2014.
Again, inspectors said the prison seemed to be locked into a negative cycle of responding to violence with punitive measures and placing further restrictions on the regime to keep people apart.
Mr Clarke said inspectors were told of large-scale fights which, he said, showed the risk faced by staff on a daily basis.
Inspectors did find some positives in the form of healthcare and described mental healthcare as impressive.
Mr Clarke said: "It would be wrong not to recognise the challenges faced by staff at Feltham A in creating a safe and decent facility.
"Violence was a serious problem and 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.
"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.
"At Feltham B, while the violence and poor regime overshadowed this inspection, despite everything, there was some very good work being carried out by dedicated staff."
The current regime must change in order to help imprisoned children turn their lives around, said Frances Crook, chief executive of the Howard League for Penal Reform.
"These are two of the worst in a long line of terrible prison inspection reports. It is all the more disturbing that they concern children and young people.
"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."
Michael Spurr, chief executive of the National Offender Management Service, said: "We are determined to improve safety and develop purposeful regimes focusing on the educational needs of the young people in our care at Feltham and across the youth estate.
"Following this inspection we reduced the number of young people at Feltham to enable the governor to develop an improvement plan and relaunch his violence reduction strategy.
"The new Youth Custody Service is focused on improving safety and performance across the youth estate, with a commitment to develop effective practice and to provide the support young people need to reform"