Imprisonment for Public Protection (IPP) sentences were introduced via the Criminal Justice Act 2003 under the Labour government.
They were designed to protect the public from criminals who had not done enough to merit a traditional life sentence, but who were deemed a threat to society at large.
One of the most famous people to receive an IPP is John Worboys, a black cab driver who police say may have raped or sexually assaulted as many as 100 women.
However, many other IPP prisoners are inside for far, far lesser crimes with no idea when they will be released. Hundreds of young men and women, many of them 18 or 19, were sentenced to indefinite terms for committing a string of non-sexual crimes with a very limited degree of violence.
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