Indeterminate sentences are a key element of the prison system. But they are both complex and controversial, but how do such sentences work?
What does indeterminate sentencing mean?
The power came in as part of so-called public protections measures in the Criminal Justice Act 2003 and has been used since April 2005.
In addition to being able to extend licence time for other crimes, judges were given the power to set indeterminate sentences.
Under this sentence, a minimum tariff for incarceration is handed down but the defendant must satisfy the authorities he or she is fit for release and does not pose any threat to the community.
Does indeterminate effectively mean life behind bars?
Should the defendant no longer be considered to pose a threat to the public, then he will be released.
Once behind bars, and having served their minimum tariff, the offender must prove through special classes and other intensive supervision, that they pose less of a risk than when they entered jail.
In theory this means that the reformed prisoner will be out when they have shown they have reformed - and those who have not addressed their offending will stay locked away.
Does it work this way in practice?
This is the big controversy with indeterminate sentences. The Prisons watchdog says there are many IPP inmates who have not been properly assessed for release because of a lack of resources and chronic overcrowding in jails.
In a major report into IPPs, the prisons and probations watchdog said that a "perfect storm" of circumstances had been created around the sentences and the system's inability to manage offenders effectively.
What do the prison reform lobby think?
There are concerns that vulnerable people who need help may find themselves locked up for years despite having not committed serious crimes. Campaigners say that children held under the younger version of IPPs are not properly assessed for risk, exacerbating their mental state.
Only 3% of IPP prisoners have been released since the scheme started, they say, and their numbers inside are growing all the time.
Paul Cavadino of crime reduction charity Nacro said: "The operation of indeterminate sentences for public protection has violated basic principles of justice. Often courts have passed these sentences after considering inadequate pre-sentence reports which overestimate the offender's risk.
"Many prisoners on IPP have been unable to begin courses that they must complete if they are to show the Parole Board they are safe to release. This indefensible situation has caused immense damage to the prison system and to the reputation of British justice."
Has the system faced legal challenge?
Yes. In 2007 the Court of Appeal said the sentencing was flawed in the case of a convict who had not been able to get on a parole course at his prison. Penal reform campaigners predict more cases will come to the courts because prison overcrowding has made the sentence unworkable - and therefore unable to establish if someone is a risk or not.
Has the government considered reforms?
The Ministry of Justice, responsible for prisons and sentencing, admitted that parts of the sentencing policy were flawed earlier this year. Legislation passed earlier in 2008 now sets out a "seriousness threshold" for indeterminate and extended sentences. This could lead to fewer repeat offenders on short sentences becoming stuck in the system without good reason. But at present, the number of IPP prisoners is still rising.