Norwich Prison has been slammed by the Prison Reform Trust for keeping 755 inmates in buildings designed for 568.
The charity's research has revealed inmates are being kept in overcrowded conditions with nearly half of the single cells holding two prisoners.
The report, called A Lost Generation, catalogues the experiences of young people in prison.
Its author Enver Solomon claims many young people spend on average 16 hours a day locked up.
Purposeful activity in prison amounts to only 18 hours a week for some inmates and this puts undue pressure on them.
A Prison Reform Trust study also says the kind of standards within Norwich Prison could have serious consequences on inmates, particularly young offenders.
Its author Enver Solomon said: "At a prison like Norwich young people will find themselves in an alien environment and will not get the care and attention they need.
"Unfortunately, those at risk of suicide will struggle to cope."