Judges have ruled that the way the government is using indeterminate jail sentences is unlawful because people are being kept in prison for longer than necessary.
Legal affairs analyst Jon Silverman looks at the implications of the ruling.
Justice Minister Jack Straw has announced a review of the sentences
The High Court judgement will have come as no surprise to ministers.
They were put on notice by the chairman of the parole board, Sir Duncan Nichol, six months ago that increased use of the indeterminate public protection sentence (IPP) by judges was having serious implications for prison management.
He revealed that official projections showed that, if unchecked, the number of IPP prisoners at any one time would reach 12,500 within five years, far outstripping the "lifer" population of jails in England and Wales.
Although the sentence was a new departure in English law - sentencing on the basis of predicted risk rather than past behaviour - the Home Office believed that it would not have a dramatic impact on prison numbers.
Officials argued that another controversial sentence, introduced by the Conservatives in 1997 - the so-called "three strikes and you're out" provision - had not significantly boosted the jail population.
But that applied to only 11 specified offences.
The IPP can be triggered by any of 153 listed violent or sex crimes, as long as the offender is deemed to be dangerous.
And given the general climate of acute concern about public safety since 2005, the courts have reached for the new sentence with unexpected enthusiasm.
That, in itself, is not the essence of the problem. The average tariff - the minimum term the offender must serve before consideration of release by the Parole Board - is 30 months.
But in many cases, it is considerably shorter. This means that, taking time on remand into account, the Parole Board is being asked to assess a significant number of offenders almost as soon as they have arrived in custody.
Self-evidently, these prisoners have not completed appropriate offending behaviour courses so they are "knocked back".
And given the sheer number of IPP prisoners - the latest government figure, given in May 2007, was 2,547 - it is impossible at present for every inmate to complete a course before their tariff expires.
'Laurel and Hardy'
The director of the Howard League for Penal Reform, Frances Crook, commented: "The IPP has been described as 'Kafkaesque'.
"Sadly, the failure of the government to prepare for the impact of this sentence on resources is more in the vein of Laurel and Hardy."
The new Justice Secretary, Jack Straw, would not concur with the language but in announcing a review of the IPP when he visited Belmarsh prison this July, he was conceding that, at the very least, a large injection of extra cash was needed to make the system work.
Some have raised the possibility that the High Court judgment has implications for all prisoners serving the equivalent of a life term, not just those on IPP, if they cannot complete an offending behaviour or sex offender treatment programme before their tariff expires.
However, it will probably take another test case on behalf of a "lifer" to determine whether that prognosis is correct.