Violence has soared at an inner-city jail fuelled by a flood of so-called "legal highs", an inspection report found.
There was an average of 32 assaults on prisoners and staff at HMP Leeds every month, watchdogs revealed, with around one in 10 resulting in serious injury.
HM Inspectorate of Prisons said levels of violence had increased significantly since the last inspection and were now double what it typically sees in local jails.
Its report added that inmates often refused to disclose the identity of assailants - suggesting they feared recriminations and lacked confidence in the system to protect them.
New psychoactive substances - commonly referred to as "legal highs" - were identified as a "major factor" in the increase.
"Despite some robust action being taken to address the challenges this presented, it was having a pervasive and destabilising effect across the prison," the report said.
Inspectors also found that levels of crowding were "very high" and the majority of cells were "poorly equipped".
At the time of the inspection at the end of last year, HMP Leeds held 1,149 adult male prisoners.
Deputy chief inspector of prisons Martin Lomas said: "This was a disappointing inspection of a prison which we assessed to have deteriorated in three of our four healthy prison tests.
"Fundamental issues around safety were having a significant destabilising impact across the prison and needed to be addressed urgently."
He added: "The new governor and his team had made a good start in getting to grips with these challenges and it was positive to see that they had a good understanding of the issues faced, as well as plans or ongoing actions to address them."
The report also found that, despite its age, accommodation at the facility was reasonably well maintained and the staff culture was "basically positive and decent".
Michael Spurr, chief executive of the National Offender Management Service, said: "Since the inspection in 2013, Leeds - like other public-sector prisons - has implemented new working arrangements to significantly reduce cost to the taxpayer.
"At the same time it has had to cope with a huge increase in the illicit supply of 'new psychoactive substances' which have fuelled instability and violence.
"Given this context, staff deserve credit for achieving good outcomes on purposeful activity and resettlement and for maintaining a positive relationship with prisoners."
He added: "Improving safety is the number one priority - and concerted action is being taken to combat illicit drugs and to provide better support for vulnerable prisoners. As the inspectorate points out, the governor and his senior team are providing good leadership and they will receive the support they need to make the improvements required."
The availability of so-called "legal highs" has been identified as a key factor in the surge of violent incidents behind bars nationwide.
New laws have been introduced to target smugglers attempting to sneak the substances into prisons.
Juliet Lyon, director of the Prison Reform Trust, said: "This is yet another inspectorate report describing a prison failing to meet people's most basic need, safety.
"How can we expect people to leave custody to be less likely to offend, if all they have experienced in prison is fear and violence?"
Frances Crook, chief executive of the Howard League for Penal Reform, said: "This grim report is a snapshot of a troubled prison system."