The credibility of the Government’s prisons policy is in the dock. For the third time since home detention curfews (HDCs) were introduced in 1999, they are expected to be extended by the Home Office. These are the orders that allow criminals serving sentences of less than four years for non-violent or non-sexual offences to finish them at home. Initially, HDCs covered the last two months of a prison term. Already, they have been extended to cover the final four and a half months. That would increase to six months under plans being considered by Charles Clarke, the Home Secretary. It would mean early release for some 1,000 extra prisoners, an increase of a third on those currently subject to HDCs. A burglar sentenced to two years could be putting his feet up at home after just six months behind bars. Residents may soon find him casing their homes or find that he has made off with the family jewels.

There may be a good case for extending curfews: the reoffending rate is 8 per cent. But the manner in which it is being considered is reprehensible. Ministers are being guided not by the merits of rehabilitating criminals, but to manage a prison system that is at bursting point. Thus, they are treating with contempt those into whose communities the delighted prisoners will return.

The prison population is, as it has been for ten years, rising. From 45,000 in 1995, it is 77,599 today. Some cells designed for one inmate already hold two and three. There are now only 700 places left in the 139 prisons in England and Wales. They could easily be filled before the end of the month.

That prisons are unable to contain safely those sent down by judges and magistrates is entirely the fault of the Government, and of Gordon Brown in particular. It has had years to plan the new generation of prisons that for a long time has looked necessary. Granted, two establishments opened last year at Peterborough and Ashford, Surrey. But because Mr Brown has refused to provide the funds, there are no more facilities on the drawing board. It will take at least three years, and longer given the planning process, until a new jail opens. The one prison ship, which for a while held 400 low-risk inmates on short sentences, has been decommissioned because it is no longer seaworthy. Increasingly, the Prison Service is resorting to “ready-to-use” units dumped in prison grounds. But these prefabricated facilities generally contain no workshop, gym or classroom facilities and thus undermine efforts at rehabilitation.

The Government appears to believe that new prisons do not win votes. This may be true. But there are votes to be lost when people scent official deceit. As in so many previous cases, the Government is trying to have it both ways. One day Tony Blair rants about banging up yobs. The next, Mr Clarke is quietly unlocking the back door and emancipating those whom society has decided to punish. Voters deserve honesty, and on this subject they have received little. The clear answer, however depressing, is to build new prisons. A first step would be a 1,500- inmate “superprison” that could house, and try to rehabilitate, all categories of inmate in a humane environment. The alternative is dire. Unless voters sense that punishments fit the crime, confidence in the criminal justice system will collapse. And unless ministers ease overcrowding, they are adding to the likelihood of a prison riot.