Paul Firth: It's not enough to cut down on prison sentences if we want to reduce crime

This review will place greater emphasis on rehabilitation. The Justice Secretary cannot be shocked to learn that those who serve short sentences often re-offend within a short time of their release. He acknowledges the risk of "a criminal underclass", a significant number of persistent offenders who refuse to be rehabilitated.

Short sentences were to be scrapped, it had been suggested. But there will always be those who insist on committing one offence after another and failing to turn up for their community order. Prison has to be available as a punishment.

I have worked with thousands of magistrates and judges. I cannot remember one who enjoyed passing a prison sentence. Phrases like "last resort" and "ran out of options" were regularly used alongside references to "serious offences" and "failure to comply with community sentences".

Famously, Michael Howard – a former Home Secretary and contemporary of Clarke – once declared that "prison works". Clarke seems to disagree with his former colleague. Or rather what he really thinks is that prison doesn't work well enough.

Although Mr Clarke recently told Newsnight viewers that he did not think a fall in crime was caused by an increase in the prison population, the statistics and Mr Howard would clearly argue against him. In the United States, for example, between 1989 and 2009 the prison population rose from 1.1 million to 2.4 million, while offences recorded by the police fell from 14 million to 11 million.

Locking up persistent offenders is not a panacea. Telling them they won't be locked up is a recipe for disaster.

Most sentences are intended either to punish or to rehabilitate. Short periods in prison provide little opportunity for rehabilitation. Longer terms of imprisonment, while "working" in Mr Howard's sense, must also look to the future.

A special wing at Feltham Young Offenders' Institution runs a unique scheme aimed at reducing reconviction rates by providing inmates with intensive support and training before and after release. The scheme has reduced re-offending dramatically. It works in Mr Clarke's sense, but it costs money.

Money for diversion from custody has always been in short supply. I remember a young man who was brought before me after he had used a baseball bat to smash the windscreens of a number of cars, moving and stationary, occupied and empty. He was of good character, sober and not on drugs.

While in custody, he had used a plastic beaker to store every tiny bit of dust and fluff he could find in his police cell. It didn't take long to decide he had a mental health problem and that, at least in the short term, he was a danger. But it took a very long time for me to find somewhere other than prison to take him.

If there is no money to provide the extra support so many prisoners need to keep them away from offending or to address their addiction or mental health problems, then how else is there to be a reduction in the prison population?

The answer is fairly obvious. Rehabilitation will outweigh punishment as an objective and fewer prison sentences will be passed. Where a prison sentence cannot be avoided, it will have to be even shorter, if need be by giving credit of up to 50 per cent for an admission made to the police.

There has always been a reduction for those who do not put their victims through the trauma of having to go to court. Under the existing guidelines, that may be up to one third of the appropriate sentence. But then any sentence up to four years is automatically reduced by 50 per cent – long gone are the days of remission for good behaviour – and some are further reduced by early release with electronic monitoring.

So, if a defendant makes an admission to the police, he could have a four year sentence reduced to two years and then have that reduced to one year or less. The reality of such a sentence seems a long way removed from what the original offence was worth. And to those not involved on the legal system, it must seem almost unbelievable.

It is obviously desirable that released prisoners should not re-offend. If they do go back to a life of crime, prison has worked only temporarily. But we have to face up to the fact that some offenders will commit crime as soon as they are given the chance. No amount of rehabilitation, inside or outside prison, will stop that underclass.

Whether they are aimed at rehabilitation or punishment, all forms of sentencing cost money. Saving money on one sentence, prison, cannot be done in isolation. There will inevitably be increased costs somewhere, perhaps even within the prison service itself.

Mr Howard also advocated "honesty in sentencing". Perhaps now we need honesty in a sentencing review. Reducing the prison population is not by itself a laudable aim. Reducing crime is.

Paul Firth is a retired district judge who sat in magistrates' courts in Yorkshire, Lancashire and Merseyside.