A boy at a young offenders institution was left to lie on a mattress on the floor of a "filthy" cell for more than 22 hours a day, a report has revealed.
The chief inspector of prisons in England and Wales, Peter Clarke, said a practice of separating children from their peers at YOIs amounted to "harmful solitary confinement".
He said the policy had "fundamental flaws" and was a risk to mental health.
The government said it would be making "immediate changes".
Inspectors looked at five YOIs, holding about 600 men and boys aged 15 to 18 - Cookham Wood in Kent; Feltham A in west London; Parc in south Wales; Werrington in Staffordshire, and Wetherby and Keppel in West Yorkshire.
Mr Clarke said 57 offenders had been separated and "in the worst cases children left their cells for just 15 minutes a day".