Safety is a "major concern" at a young offender institution rife with drugs, violence and gang-related activities, an inspection report warns.
HM Inspectorate of Prisons (HMIP) found that HMYOI Aylesbury had deteriorated further in a number of areas since it was last inspected in 2015.
Inspectors also found a "significant increase" in prisoner violence against both staff and other inmates.
The facility holds around 440 young adult men, mostly aged from 18-21.
HMIP's report was based on visits to the Buckinghamshire establishment in April; in the previous six months, there had been 38 assaults on staff, 133 on prisoners and 77 fights.
Thirty-three assaults were classed as serious, often involving multiple assailants and a single victim.
The watchdog also found that prisoners reported "high availability" of drugs and alcohol.
The chief inspector of prisons, Peter Clarke, said: "We found a combination of volatile and frustrated young people, too few staff and many who were inexperienced, and prisoners locked up for long periods with no activity and too little sentence progression.
"These factors led inexorably to some poor outcomes. In particular, safety was a major concern."
He said HMYOI Aylesbury "showed some areas of considerable potential", adding: "Most staff appeared remarkably resilient and wanted to improve the prison."
Michael Spurr, chief executive of HM Prison & Probation Service, said: "The chief inspector is right to praise the professionalism and resilience of staff at Aylesbury.
"This is why additional staff are now being deployed to Aylesbury from other establishments to provide a consistent regime for prisoners and there are firm plans in place to fill vacancies through permanent recruitment."