Barely five months ago, Jack Straw said he couldn't build his way out of the prisons crisis and wouldn't try to; this week, in a new political climate, he announced his intention to do just that.

In his time as home secretary, he succeeded in stabilising prison numbers, if not in making imprisonment the punishment of absolute last resort. With prisons back on his plate, this time as lord chancellor, Jack Straw confidently indicated that the only way pressure could be relieved was by sending fewer people to jail and using more non-custodial sentences.

As prison numbers spiralled out of control, and the haste of badly drafted, headline-grabbing legislation was exposed, a consensus seemed to be emerging that a diet of tough laws and more prisons was just not sustainable or sensible.

Sadly, hopes that we would see a new, authoritative approach from the Ministry of Justice were dealt a pretty savage blow this week by its response to prisons reviews by Lord Carter and Baroness Corston.

Despite some welcome fine-tuning on scope for more proportionate sentencing, Lord Carter's government-commissioned review of the future of prisons was ultimately a consultant's report on managing a storage problem. It raised few questions, and offered fewer answers, about the purpose of prison or its social and economic outcomes.

Yet the sheer scale of imprisonment and the impact it has on prisoners, their families and communities, is staggering. Each year, more than 132,000 people go to jail and 70,000 children enter the youth justice system. During their school years, an extraordinary 7% of children will experience their father being imprisoned.

Our prisons have become a dumping ground for the socially excluded. So much of the pressure on prison places could be eased if the government commitment to reserve prisons for serious and violent offenders turned from woolly aspiration to a steely determination to stop prisons lurching from one crisis to the next.

Jack Straw's acceptance of Carter's plans for super-sized, so-called titan, jails flies in the face of experience. Everyone knows that giant institutions don't work, whether they are schools, hospitals or prisons.

It is the biggest prisons - half the size of the proposed titans - that cause the biggest problems at the moment. Important safety concerns have already been raised by current and former chief inspectors of prisons and the Prison Governors' Association. There is also the risk that, having first call on scarce resources and even scarcer ministerial attention, these titans will engulf any sensible plans to reform the justice system.

It is difficult to see how pouring billions of pounds of public money into giant prefabricated warehouses, each holding over 2,000 people, will give hard-pressed prison staff the chance to work intensively to reduce reoffending. On current form, the end result is going to be armies of ex-offenders released homeless, jobless, out of touch with their families and ready to offend again

Investment instead in treatment for addicts would lead to a dramatic drop in offending. Most acquisitive crime, such as shoplifting and theft, is driven by drugs, while binge drinking fuels violence and public disorder offences.

The government's preparedness to consider diverting the mentally ill from police stations and courts into much-needed healthcare is welcome, at least. The fact is, so many of the solutions to crime lie outside prison bars.

That is the thrust of the approach advocated by Baroness Corston when she was commissioned to report to ministers after the tragic deaths of six women in Styal prison. She couldn't see the point of, at huge cost, locking up vulnerable women offenders who pose no risk to the public. Instead, she called for the closure of women's prisons over a 10-year time period while government established some small custodial units for dangerous offenders and a larger network of support and supervision centres.

Based on existing successful community centres that I visited as a member of the Corston review group, these would provide access to services to help women deal with addictions, mental illness, rape and domestic violence trauma and debt, while they would also gain skills and take responsibility for their families.

The response to Baroness Corston's review was promising, on the face of, it but ultimately insubstantial. Ministers agree with her analysis of the problem and nearly all her recommendations. Indeed, they have been saying as much since 2001. It's just the making-it-happen part of it all that they cannot support. So, no to money and no to the Women's Commission that is needed to drive things forward.

It's difficult to understand this reluctance to match words with deeds. Years of inaction to help some of the most vulnerable people in our society is as unforgivable as it is inexplicable.

The number of women in the system is small. The new economics foundation has just submitted to ministers interim findings showing how the Corston recommendations could produce significant cost savings. And last week's report by SmartJustice, with an ICM poll, revealed overwhelming public support for alternatives to prison for non-violent offenders.

Fudging their response to Corston, ministers have gratefully grabbed the Carter report and borrowed money from the next spending round to bail them out of the hole they are in. But it will only drop them, or their successors, in a bigger hole if they continue to act as custodians of social dustbins rather than as leaders of a fair, effective justice system.

PRT's latest Bromley briefing prison factfile is available to download here