Large amounts of rubbish outside Winchester Prison led to a jail rat infestation, a report found.
Between June 2014 and May 2015, the Independent Monitoring Board (IMB) said the prison was under "considerable external pressures".
It also said there was increased use of so-called legal highs and violent behaviour.
A rat infestation had "caused problems inside and outside, including the destruction of CCTV cabling".
Prisoners were throwing rubbish out of their cell windows, allowing it to collect in outside areas, the report said.
"This year there has been a rat infestation, perhaps as a consequence, which made working conditions in control, located in part of the oldest building, almost unbearable," it added.
The report also stated "many prisoners spend their time in the workshops spinning out what little work there is or simply sitting chatting".
It also said staffing was "highly stretched", and found an "increasing use of new psychoactive substances" amongst inmates.
However, the IMB said there had been an influx of prison officers from elsewhere, with high levels of training.
It also said a new catering academy was due to launch which would provide qualifications, and praised the carpentry department for teaching in an "atmosphere of friendly mutual respect".
During the period the report covers three prisoners died in custody - one from suicide and two from natural causes.
Since the report, BBC South Today found there have been six further deaths - at least two are thought to have been due to natural causes.
In a statement, the MoJ said it took a "zero tolerance approach" to drugs in prison, and added there was "already a range of robust measures in place at Winchester to detect drugs", including the use of search dogs trained in detecting new psychoactive substances.