R v Watty [2007] EWCA Crim 123, [2007] All ER (D) 219 (May)
In order to pass a sentence of imprisonment for public protection, the judge has to be satisfied that there is a serious risk of the defendant causing serious harm by the commission of further offences.
The test requires: (i) that there is evidence to conclude that there is a significant risk of the defendant committing further offences of the kind which might give rise to physical or psychological damage; and (ii) that there is a significant risk of his causing serious harm by so doing.
Repetitive sexual offending with relatively low or without serious harm does not of itself give a significant risk of serious harm in the future. Parliament cannot have intended a sentence of imprisonment for public protection for offences of a relatively minor nature which would at most result in a short term of imprisonment.