A young offenders institution was found to be "not safe for either staff or boys" after an unannounced inspection.
The HM Inspectorate of Prisons (HMIP) gave YOI Feltham in west London its lowest judgement for safety after finding levels of "very serious" violence had risen.
The prison on the same site was also found to have had a "significant increase in violence".
However, staff were also praised for "many examples of good work".
Feltham A holds boys aged 15 to 18, while Feltham B is a prison for young adults aged 18 to 21.
Chief Inspector of Prisons Peter Clarke said violence at the young offenders institution was a "serious problem" and "a serious assault on an officer" had happened while the inspection took place.
As well as a rise in violence, inspectors also criticised how long boys were allowed out of their cells as a result of a strict but "ineffective" regime used to manage behaviour.
About 40% of boys were locked up during the school day while 30% were allowed out of their cells for only two hours each day.
This meant some were prevented from being able to use basic amenities including showers and telephones, inspectors said.
Feltham B was also found to be an "unsafe environment" with inmates "often afraid for their own safety".
"Prisoners were enduring a regime that was unsuitable for prisoners of any age, let alone young men," inspectors said.
Nevertheless, staff at both the YOI and prison were praised, with healthcare - particularly mental health provision - described as "impressive".
Work to resettle offenders back into the community upon their release from both institutions was also considered to be "generally good".