Prisoners moved from London to a jail on the Warwickshire border have been blamed for a "sharp rise" in violence.
HM Inspectorate of Prisons found standards have declined at HMP Onley and declared it unsafe.
The watchdog found conditions worse than in 2012, with staff blaming relocated London prisoners and gang issues for a near-tripling of assaults.
The National Offender Management Service said tackling the decline in safety was the governor's top priority.
The category C prison, near Rugby on the Warwickshire and Northamptonshire border, held about 740 prisoners at the time of its inspection, from 25 July to 5 August.
Since its previous inspection in 2012 it has been designated as a resettlement prison for Greater London.
Inspectors found there had been a "dramatic decline" in standards in that time.
"Staff gave various explanations, including the change of prisoner population and gang-related issues that they brought with them, the impact of new psychoactive substances [previously known as "legal highs"], and the impact of reductions in staff numbers," the report said.
But despite the rise in violence, "not enough had been done to analyse the root causes".
Peter Clarke, chief inspector of prisons, acknowledged good work being done at the prison but called for the leadership to "get a grip" and "halt the decline".
Staff shortages "did not offer an excuse for a decline in standards of the severity that we found", he added.
Michael Spurr, CEO of the National Offender Management Service, welcomed the inspectors' recognition of work being done at the prison.
Additional staff were being recruited to drive forward improvements required, he said.
- No comprehensive violence reduction or drug reduction strategy
- Existing drug reduction strategy did not specifically address the problem of new psychoactive substances (NPS), which were having a significant impact
- Massive backlog of security-related information reports undermined a proactive approach to violence
- Staff shortages had contributed to a restricted regime for prisoners
- Offender supervisors often had limited contact with prisoners
- Most prisoners did not have an up-to-date risk assessment
- Staff and prisoner relationships were reasonably good, as were health services
- Range and quality of education and training was good
- Support for prisoners to resettle was mostly good