A rat infestation at a failing prison was so bad inmates were forced to block gaps under their cell doors with cardboard and towels, a report has found.
The smell of rat urine was "overpowering" in some areas of HMP Rochester in Kent, chief inspector for prisons Charlie Taylor said.
The report detailed the "squalid" and "decrepit" conditions, rising violence, self-harm and widespread drug use at the prison.
The conditions have led the watchdog to put the lower security jail into special measures and call on the justice secretary to make improvements.
Rochester prison had been condemned for closure in 2017 but remained open due to the overcrowding crisis in jails, Mr Taylor said in the latest inspection in August.
Drug use at the category C prison, which holds more than 700 men, was "endemic" and there was a growing number of attacks on staff, the report said.
The jail was "fundamentally failing in its rehabilitative purpose as a category C training and resettlement prison," with some of the worst conditions inspectors had seen in recent years.
"Staff and prisoners told us that rats and mice regularly entered cells and offices on the older wings," the report said.
"Prisoners resorted to creating barriers from cardboard or towels to fill gaps under cell doors to keep vermin out."
It added: "The smell of rat urine in some areas was overpowering.
"Throughout the inspection, both staff and prisoners complained about the infestation.
"Staff showed us where the rats had come in through ducting and around pipes.
"It was evident that mice were also commonplace, and on some wings nearly all prisoners put makeshift barriers beneath their cell doors to try to keep rodents out."
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Andrea Coomber, chief executive of the Howard League for Penal Reform, said: "This report into Rochester prison underlines the terrible situation that the current government inherited on taking office.
"But it also underlines the fact that government ministers can only use that as an excuse for so long."
Despite welcoming recent efforts to tackle problems in prisons, she warned: "If we are still seeing inspection reports like this in a year's time, then the government will have only itself to blame."
Security measures will be reviewed, cells are being refurbished and more effort will be made to get prisoners into training and education, a Ministry of Justice spokesperson said.