Safety is a "major concern" at a young offender institution rife with drugs, violence and gang-related activities, an inspection report warns.
HMYOI Aylesbury had deteriorated further in a number of areas since it was last inspected in 2015, the assessment found.
The facility in Buckinghamshire holds around 440 young adult men mostly aged between 18 and 21.
In a report based on visits to the establishment in April, HM Inspectorate of Prisons (HMIP) said there had been a "significant increase" in prisoner violence against both staff and other inmates.
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 report warned that the impact of gangs on violence in the prison was not yet fully understood.
It noted that the security team had developed a database to "handle the complexities of gang management", with prisoners said to have had links to around 118 gangs.
The watchdog also found that prisoners reported "high availability" of drugs and alcohol.
Between October 2016 and March 2017, there had been 59 drug finds plus 226 discoveries of mobile phones.
HMIP also raised concerns over living conditions at the institution, saying many residential units were in poor condition and basic standards of decency were not being achieved.
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.
"They do some remarkable work with young adults serving long sentences who have complex needs and whose behaviour can be very challenging.
"Improving safety and addressing ongoing staffing challenges remain the Governor's top priorities.
"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."