Concerns have been raised about a "disruptive" change to use a Kent jail as an adult prison and a young offender institution (YOI).
The independent monitoring board for Rochester prison and YOI also said the change did not appear to deliver "any discernible benefits".
Board chairman Euan Eddie also said the move was an unnecessary and unwelcome diversion of management and staff time.
The Prison Service said the report would be considered by the government.
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