Levels of violence at a young offender institution have dropped after teenagers were rewarded for good behaviour - including being given sweets and chocolate.
Inspectors found an 80% reduction in assaults on staff at HM Prison and Young Offender Institution (YOI) Feltham from a previous visit early last year.
At the time of that inspection, violence was found to be a serious problem and the unit in west London was judged to be unsafe.
By the time inspectors visited again in January this year, safety had improved "quite dramatically".
According to a report by HM Chief Inspector of Prisons, Peter Clarke, the progress was in part due to a "new behaviour management philosophy".
He wrote: "Last year we reported how the focus had been on sanctions and regime restrictions; there was a cycle of violence and punitive responses, with no obvious strategy in place to break it.
"This had changed, and on this occasion we found a new focus on rewards and incentives for good behaviour."
The young people at the prison took part in a merit scheme, the report said, where points for good behaviour could be exchanged for confectionery at a "merit shop".
Inspectors described the scheme as "motivational", and said the inmates had spoken of it positively.
The report also praised the fact teenagers were eating meals together and spending more time out of their cells, after the previous inspection found inmates ate alone and were locked up for almost 20 hours a day on average.
Michael Spurr, chief executive of Her Majesty's Prison and Probation Service, said: "I am very pleased that the chief inspector has recognised the hard work of the governor and his staff at Feltham, and have acknowledged the real progress they've made.
"Improvements in safety are a step in the right direction and it is always rewarding to hear staff praised for being patient, enthusiastic and dedicated."
Justice minister Phillip Lee added that it was "encouraging" to see the progress made "under very challenging circumstances".
Inspectors also said, however, that a large number of looked-after children at the unit, which held 140 mainly 16 to 17-year-olds at the time of the inspection, were not always receiving sufficient support from local authorities, particularly on release.
The report also warned work was needed to address gambling issues, after some inmates reported it was possible to get into debt at the prison.