A major young offenders' institution is suffering "unpredictable and reckless" violence, prison inspectors say.
They warn that staff at Feltham in west London are constantly trying to keep apart boys from more than 40 gangs.
Too many of the prisoners were locked in cells for up to 23-hours a day, inspectors found.
The report comes amid a warning from union officials that jails in England and Wales are so violent they are being controlled on a "wing and a prayer".
In a report looking at conditions at Feltham, chief inspector of prisons Nick Hardwick said there had been improvements but serious concerns remained.
The report by Her Majesty's Inspectorate of Prisons (HMIP) said there was continuing violence between 240 boys at the Feltham A unit, which is dedicated to 15 to 18-year-olds.
The watchdog said that although the number of fights and assaults had fallen since its last official inspection, the total remained too high and many incidents were gang related.
In the six months before the inspection there had been 262 violent incidents, including 79 assaults on staff.
At the time of the inspection, 48 gangs were represented at the unit, meaning that staff were involved in "constant juggling" to ensure boys from rival groups were kept apart.
Locking them inside their cells for much of the day amounted to solitary confinement, the inspectors said.
Mr Hardwick said the fall in the number of boys in custody nationally meant that those who remained were a more "concentrated mix" of troubled youths.
"As at other YOIs [Young Offender Institutions] for this age group, staff in Feltham A still struggled to manage this behaviour in a safe and secure way," he said.
"Staff need more help to do this and I repeat my call for the Youth Justice Board (YJB) to initiate an independent expert review of its policies and resources for managing behaviour, reducing bullying and supporting victims across all YOIs."
Michael Spurr, chief executive of the National Offender Management Service, said: "As this report makes clear the challenge faced by the governor and staff at Feltham should not be underestimated.
"There is no easy answer to the challenges presented by the young men in Feltham but we are committed to working positively with our partners."
The report echoes problems across the prison estate, detailed in an interview with the Prison Officers Association for BBC Radio 4's File on 4, to be broadcast on Tuesday.
The number of serious assaults on inmates and staff reached a record 1,800 in the year to June 2014 - but the union says its members are struggling to cope because of job cuts.
Association chairman Peter McParlin said: "We've always had incidents of assaults in prisons, but the level and the numbers now are out of anything that I've experienced in the last 32 years.
"You effectively manage it on a wing and a prayer. I wake up every morning thinking today is the day one of my colleagues will be murdered in their work."
Ministers blame the escalating violence on a rise in the number of men jailed for violent offences and say they have taken steps to ensure prisoners are prosecuted and punished if they commit crimes behind bars.