Dominic Lawson: Our system of justice is not just rotten – it is lethal

How can it be possible for one probation officer to handle 60 "clients" at any one time?

Here's a question for you. What proportion of those charged with murder are men under the supervision of the probation service? The answer, as released over the weekend by the Shadow Justice Secretary, Dominic Grieve, is: one in seven. Mr Grieve argues that this figure, based on the past three years' statistics, demolishes Jack Straw's claim that the horrifying murder of two French students by a highly dangerous probationer, Dano Sonnex, was a "one-off case".

The Justice Secretary, naturally, is able to bat back this statistic with one of his very own. He pointed out that less than one offender in 200 commits a 'further serious crime' while under the supervision of the probation service. In strictly utilitarian terms, this provides us with an interesting calculus: how do we balance the misery caused by "serious" crimes committed by probationers, against the happiness given to those large number of prisoners released before the conclusion of their sentence?

Like all utilitarian calculi, it is impossible to resolve, since happiness or wellbeing is not divisible into mathematical units. Take the fact that in the two years to October 2008, over 100 men under the supervision of the probation service were convicted of rape – convicted, that is, not merely accused. How do you measure the misery, trauma and pain of the victims of those more than 100 re-convicted rapists? And how do you balance that against the happiness given to those men released ahead of the end of their sentence for some earlier rape, and who have not re-offended?

Nevertheless, there is a mathematical calculus underlying all this pain and misery: the Government sanctions a system which automatically releases prisoners well before the completion of their sentence, purely to save money. It costs close to £30,000 a year to keep an offender in prison, which is very much less than what the state spends on his care via the probation service, once on the outside.

Somewhere in Whitehall, there must be a civil servant who has worked out what is the "acceptable cost" of a murder or rape. It would be worth making an enquiry under the Freedom of Information Act, to find out what that figure is. It would provoke a debate which should be mercifully free of the usual cant from ministers about how every murder or rape committed by a prisoner released early is "unacceptable". It clearly is acceptable to them.

In the Commons debate a week ago on the Sonnex case, Mr Straw told the House that: "In 2003 Sonnex was sentenced to eight years imprisonment for multiple offences. He behaved violently in prison and admitted to a prison medical officer that his 'reactions could kill'. He was released from prison on 8 February 2008, the latest date he could lawfully be held in custody."

Yes, those figures are correct. You will already have worked out that they are very odd. If Sonnex was sentenced in 2003 to an eight-year stretch, how could less than five years be the absolute maximum that he could "lawfully be held in custody"?

The answer is that this Government ( and, to be fair, it is following its Conservative predecessor) release even the most dangerous non-lifers automatically when they have served two thirds of their sentence in custody; and those given sentences of 4 years or less are released by executive order when they have completed just half of their prison sentence. This is nothing to do with rehabilitation, since it is automatic – based purely on cost, and, I suppose, a fear of rioting if the prisons become even more overcrowded: in other words, the risk is passed from prison officers to the unwitting general public.

Can it be argued that the judges have passed initial sentences which have not taken account of the overstretched nature of the prison service, and that therefore it is reasonable to have an automatic early release? Well, first of all, we should hope that our judges pass the sentences which they believe fit the crime; secondly, it so happens that the Compendium of the Sentencing Guidelines Council states that: "In view of the dangerous overcrowding of prisons, where a sentence of imprisonment is necessary, it should be as short as possible, consistent with public protection." In other words, the initial sentence handed down by judges is already designed to take account of the pressure on our prisons: the further halving ordered by the Government is a kind of double counting (or rather, double-halving).

The supporters of early release point out that if it is the case that a prisoner has a high likelihood of re-offending, then by keeping him inside for longer we would only be deferring the harm he will do when released, not preventing it. I should like them to try making that point in person to the parents of Laurent Bonomo and Gabriel Ferez – something along the lines of: "We are so sorry that Dano Sonnex and his accomplice stabbed your sons over 200 times, but please understand that if we had released Mr Sonnex three years later, he would probably have done the same thing to someone else's children in 2011, which would mean that those children's parents would suffer no less than you are now doing."

In any case, there is a further statistic, which demonstrates that even this argument can not be made with confidence. A Ministry of Justice bulletin last month observed: "For offenders discharged from custody, the frequency rate falls as the length of sentence increases. In the 2007 cohort, offenders with a sentence length of less than one year had a frequency rate of 303.8, compared to 56.9 for offenders with a sentence of four years or more."

Stripped of jargon, what this means is that someone given a sentence of more than four years is almost six times less likely to re-offend than someone who has been given a sentence measured in months. When you consider that the man given the longer sentence is likely to have been a more habitual offender in the first place, this is an eloquent measure of prison's deterrent effect, at least on those who have already experienced its privations.

As a result of the Sonnex case, the head of the London Probation service, David Scott, resigned. This was the head on a platter which Jack Straw was able to provide to the Commons. Yet Mr Scott's job was an impossible one. Mr Straw told MPs that Sonnex was one of 120 cases being handled by one probation officer, and that "for the level of workload and responsibility, it should have been 50 or 60." Oh, should it? How can it be possible for one probation officer to handle 60 "clients" – many of them very dangerous people – at any one time? Yet that is the inevitable consequence of the vast number of prisoners released early on parole – based on the idea that one twenty minute conversation a month with a probation officer is sufficient to ensure the public's protection.

I sometimes think it would be better to close down the entire probation service, and spend the money saved on more prison spaces. It would at least be a more honest system than that which betrayed Laurent Bonomo and Gabriel Ferez.