Chronic staff shortages, food running out and a surge in violence were among the findings of a critical prison report into Wormwood Scrubs.
Inspectors also found areas of the west London jail were strewn with litter, attracting rats and cockroaches.
Chief inspector of prisons Peter Clarke said the findings painted an "extremely concerning picture".
The Ministry of Justice (MoJ) said the jail was recruiting staff in a bid to "urgently" raise standards.
The HM Inspectorate of Prisons' report said Wormwood Scrubs, which holds more than 1,200 men, had high levels of often serious violence, resulting in some significant injuries.
It also detailed how food routinely ran out in one wing, with staff having to source a half-used tray from another servery or distribute "mountain survival" dried food packs.
- There had been a "dramatic" increase in violence against staff, with more than 90 assaults in the six months to July, when inspectors visited the jail
- Far too many windows facing the perimeter wall were broken, which enabled prisoners to retrieve contraband thrown over the wall
- The prison stores had not been open for many weeks, leaving staff to "scavenge" for many basic items needed by inmates
- A case sample found important aspects of public protection work had not been done, potentially leaving some serious issues unmanaged
Publishing the latest assessment, Mr Clarke said: "Wormwood Scrubs is an iconic local prison serving communities in London.
"Overall, this was an extremely concerning picture, and we could see no justification as to why this poor situation had persisted since 2014.
"The governor and his team were, to their credit, working tirelessly to address the problems faced."
The MoJ said the prison had taken "decisive action" to reduce violence and was working to urgently improve conditions.
A spokesman said: "We know staffing remains an issue, so we are recruiting 120 extra officers and will cut the time taken for new recruits to begin training.
"The addition of new, senior probation staff has also led to significant improvements in resettling offenders into the community following release.
"We are pleased inspectors recognised the hard work and dedication of staff at the prison, especially in improving education and purposeful activity."