Safety levels at a young offenders institution dropped to an "appalling" level over the first half of the year, inspectors have said.
"Fundamental change" was required at Feltham YOI in west London following a surge in violence and self-harm over six months, it was found.
Inspectors visited part of the prison known as Feltham A in July following an unannounced check in January.
The Youth Custody Service said "immediate" changes had been made.
In August Justice Secretary Robert Buckland announced a series of pledges to improve standards at the prison.
Feltham A holds children aged between 15 and 18 who are on remand or have been convicted.
Prison inspectors found there had been a "dramatic decline" in safety levels despite the number of inmates falling from 148 to 108.
In six months, there was a 45% rise in violence incidents, while levels of self-harm had tripled and the number of assaults against staff surged by about 150%, according to the report.
Some 74% of children said they had been physically restrained at Feltham A, while 40% claimed they had felt unsafe at some point during their stay.
Chief inspector of prisons Peter Clarke said the "appalling situation we found cannot be allowed to continue".
"Short-term improvements followed by dramatic and dangerous declines should no longer be tolerated," he said.
Mr Buckland's pledges to improve standards, including more staff training and repair work for cells, were made after Mr Clarke highlighted his concerns to the justice secretary.
Helga Swidenbank, executive director for the Youth Custody Service, said prisoners could "now access a full regime and there has been a sustained reduction in levels of self-harm, the use of force and the number of assaults on staff".
"The full range of improvements will need time to take effect, but progress is being made," she said.