The law of unintended consequences is not on the statute book but appears to exercise enormous influence over British penal policy. There can be few more shining examples of this than the fate of indeterminate sentences for public protection (IPPs), whose credibility was brought into severe question by a landmark ruling on Monday. If the Ministry of Justice does not act swiftly, it will find itself in the ludicrous position of not only having to release from prison individuals whom it would be proper to keep there but also offering recompense for their incarceration.

The IPP was a response to a serious law and order dilemma. Large numbers of people who had committed crimes that were serious but below the threshold for life imprisonment were being released as their sentences ended, despite the strong suspicion that they would offend again. Ministers were advised that they could best deal with this category of criminals by creating what, in effect, was a threshold below that of life imprisonment but not dissimilar to it. From 2005 an indeterminate sentence could be imposed on a person who committed any one of 153 offences (mainly connected to sexual criminality or violence against the person) where the offence carried a maximum prison sentence of ten years or more and if the person convicted was deemed by the courts to be dangerous.

In these instances, a minimum sentence would be set by a judge, after which a parole board would decide whether the person involved remained a menace to the public. If prisoners could show by means of successful completion of courses relating to past behaviour (those designed to tackle alcohol abuse and anger, for example) then they could be released on licence. If not, they would remain behind bars indefinitely but with the opportunity to bid for their release every two years.

The assumption behind this policy was that a comparatively small number of serious offenders would be awarded relatively long minimum sentences. The practice, though, is that the judiciary has seized the chance to hand the difficult decision as to how long to imprison awkward characters back to the parole boards and their advisers. More than 3,000 people are presently in prison courtesy of an IPP but with an average minimum tariff of just 30 months. Parole boards today are being swamped by applications, and the system cannot provide the courses and, consequently, the assessment required. Inevitably, this mess is becoming the basis for premature release and compensation claims.

Ministers have little choice but to embark on a crash programme of course provision. This might be enough to deal with the immediate situation but it may threaten fresh disasters. There has to be the risk that courses provided at breakneck speed will be used to judge prisoners after they have spent a fairly short time in prison will not be sufficiently sophisticated. They may reject those who apply on the basis that there has not been time for them to overcome their demons or, particularly when everyone involved with the penal system appreciates that the cells are virtually full, the courses may become an alibi for early releases. Autonomy over sentencing could be in the process of passing from judges to anger-management experts. This would scarcely be an outcome that the public should be relaxed about.