Rising tension has left a prison "teetering on the edge of a major incident", a report has warned.
HMP Winchester in Hampshire has seen increases in self-harm, drug use and violence, the prison's Independent Monitoring Board (IMB) said.
Its annual report said only the "resilience" of staff had prevented serious disruption.
The Ministry of Justice (MoJ) said it had taken "immediate action to improve safety".
It said it had introduced body-worn cameras in the prison, as well as installing additional CCTV and recruiting extra staff.
The IMB said the Victorian jail was "understaffed", with "inadequate facilities".
Some inmates were locked up for at least 23 hours a day in small shared cells, creating "frustration", the report said.
The "ageing fabric and condition" of the building was deteriorating, with limited disabled access.
The IMB said the prison's "inhumane" care and separation unit was "a dank, dark unpleasant dungeon" and had been repeatedly recommended for closure.
It said the deaths of four inmates during the past year had highlighted "ineffective management of highly vulnerable prisoners".
Levels of violence were "high", with about 17 attacks by prisoners recorded every week.
The IMB said the arrival of a new governor in August 2016 had had "an increasingly positive impact".
A smoking ban introduced in January had led to "the introduction of tobacco as a competing contraband with other drugs", the board added.
The report, covering the period from June 2016 to May 2017, said prisoners were not well-prepared for release.
It said: "If the MoJ would get education and job opportunities right it could dramatically reduce the future prison population."
In a statement, the ministry said the prison had taken measures to improve security.
It added: "The board have rightly recognised some improvements, following the appointment of a new governor, but there is much more to be done.
"That is why the governor introduced changes to the regime in August which has resulted in a more stable environment for staff and prisoners."