Prison doesn't work. So why are we locking up so many people?

"For the last three weeks there has been an utterly inexplicable rise in the prison population of about 600 prisoners a week." The words of Martin Narey, the director-general of the Prison Service, ought to prompt more than the customary political panic. By this morning the prison population, which seemed to have stabilised at around 45,000 in the late 1980s and early 1990s, will have passed the 70,000 mark. Although the causes of the surge of the past three weeks have baffled those who might know, the underlying trend, a rise in the number of prisoners of 5,000 since this time last year, is utterly explicable, and indeed was predicted.

Prison policy in the UK has been an uphill slalom for the past half-century. Politicians, under pressure to do something about crime, pass laws and write guidelines requiring more criminals to be imprisoned for more crimes for longer. Every now and then, however, there is a crisis as the walls of our prisons begin to bow under the weight of numbers contained therein. Politicians then scrabble around for "tough" alternatives to jail, which they try to sell to a sceptical public, educated only in a simple lock-'em-up logic that reached its conclusion in the bone-jarring stupidity of Michael Howard's "prison works" slogan. The prison-building programme then catches up with the prison population, and we resume the zig-zag which has given us the least successful prison policy in Europe.

The election of a Labour government committed to being tough on the causes of crime was a chance to break this self-defeating sequence. Unfortunately, Tony Blair's first Home Secretary, Jack Straw, lacked the courage to seize it. David Blunkett is a more adventurous figure and shows some signs of being willing to challenge the assumptions of the past, but he still poses too often for the punitive gallery.

At least he will have no difficulty in rejecting a demand from the Prison Governors' Association to take away from magistrates altogether the power to hand down custodial sentences. But will he use this crisis to ask deeper questions about why so many people are in prison, and whether they should be?

One of the causes of the rise in the prison population in the past seven years has been the growth of illegal drug use, with crack cocaine a particular problem. This week's suggestion that shock tactics should be used to discourage young people from trying heroin is off the point. Information is important, but the interplay between the desire to escape reality and addiction is more complex than a simple Pavlovian model. Medicalising addiction rather than keeping it in the criminal underworld offers the best hope both of controlling hard-drug use and of limiting the effect on crime.

If people who have problems with illegal drugs, alcohol or mental health were taken out of our prisons, there would be hardly anyone left. When it comes to the wider causes of crime, however, one of the best places to start is prison itself. With 58 per cent of inmates recorded as reoffending within two years of release, prison clearly does not work. A brave Home Secretary would conclude that prison obviously reinforces many of the causes of crime. It breaks up families and usually gives prisoners nothing better to do than plan how not to get caught next time.

All parties welcomed the Woolf report of 1991, yet none has acted on it to improve the education service in prison. If schoolchildren can have life coaches, why not prisoners? Instead, education, anger-management courses and anything constructive are often the first things to be cut when governors come under pressure to save money.

Our prison system is an affront to a civilised society, as is witnessed by the number of legal actions brought by prisoners under the Human Rights Act. Even if the voters did not care about that, they should care that it is a very expensive way of training criminals for more crime. Many of the alternatives to prison may not result in much better reoffending rates, but at least they are cheaper.

Mr Blunkett should make the radically pragmatic case for the humane treatment of the nation's criminals as a first step in tackling the causes of crime.