The Prison Reform Trust says young people at a Leicester institute spend more time in their cells than inmates at other sites across the country.
HM Glen Parva Young Offenders Institute has been rated one of the worst for leaving inmates locked up.
A report ranked it 41 out of 48 as it only occupied inmates in useful activity for 17 hours a week.
A spokesperson for the institute says new initiatives have recently been introduced to address these concerns.
The trust report criticised the prison service for both the high amount of time inmates spend in cells and for the lack of time spent in "useful activity" at Glen Parva.
Statistics for 2003-2004 show its inmates spent 17.3 hours a week in useful activity, and an average of another seven hours out of their cells.