Figures on safety in prisons show that there were a total of 3,634 assaults in young offender institutions (YOIs) in 2011, compared to 3,115 in 2010, an increase of 16.7 per cent.
The biggest increase came at Ashfield YOI near Bristol where the number of assaults nearly doubled from 526 in 2010 to 1,039 in 2011.
Out of the eight YOIs where boys are held, the number of assaults decreased at just two. Assaults at Warren Hill in Suffolk fell from 350 in 2010 to 209 in 2011, and from 138 to 61 at Cookham Wood in Kent during the same period.
Andrew Neilson, director of campaigns at the Howard League for Penal Reform, said the figures highlighted ongoing concerns about Ashfield YOI.
“Ashfield prison continues to be dogged by allegations of discrimination, the overuse of segregation, restraint and the handing out of additional days to children as a punishment,” he said.
“Statistics now show that in addition to all of this the prison has suffered the highest number of assaults in the entire youth estate. The number of assaults has nearly doubled in one year and Ashfield accounted for nearly seven per cent of the total assaults for the entire prison system, both juvenile and adult.
“As a private prison, it would appear that Ashfield’s primary concern is jail children for profit rather than ensure their safety. If Ashfield cannot provide a safe environment then this is bad news for all of us, as children will be released into our communities more accustomed to violence than ever.”
A Prison Service spokeswoman argued that the number of "assailants", "fighters" and "victims" had all decreased in the 15-to 17-year-old age group between 2010 and 2011, despite the rise in assault incidents.
"Prisons have a responsibility to keep prisoners, staff and visitors safe and the management of violence and its reduction is central to successful prison management," she said.
"The National Offender Management Service (NOMS) and the Prison Officers Association (POA) are jointly committed to a zero tolerance approach to assaults. Whilst we cannot prevent every incident we are confident that our systems are robust, staffing levels are adequate and suitable support is given to staff and prisoners if they are affected in any way by violence."