Dominic Lawson: Both parties say they are the toughest on crime. So where are the extra prisons?

Watching Blair and Cameron was like observing two eunuchs fighting over a pack of Viagra

It is hard to find words adequate to describe the spectacle of Tony Blair's Commons spat with David Cameron, as each of the party leaders attempted to prove he was "tougher on crime" than the other. But let me try: it was like watching two eunuchs fighting over a pack of Viagra.

The proximate cause of their preposterous squabble was the fact that a sentence passed last week on a highly dangerous paedophile for the abduction and sexual assault of a three-year-old girl allowed for the man's release after five years of a "life sentence" of 18 years. In fact Judge John Griffith Williams made it clear that Craig Sweeney "should only be released when there is no risk of him reoffending".

But the mathematics of the Government's own sentencing guidelines are clear. Although Judge Williams declared that the crime merited an 18-year stretch, under the 2003 Criminal Justice Act Sweeney would be entitled to parole after serving half his sentence. That makes nine years. Take away a further three years - the same Act insists judges reduce sentences by a third if a defendant pleads guilty - and a further year off for time spent on remand, and you arrive at the "five-year" sentence that has convulsed the body politic.

It was profoundly cynical of John Reid, the Home Secretary of the month, to blame Judge Williams for following the mandatory guidelines set by this Government. But that at least was openly opportunist. Not so the disgusting deviousness of New Labour's backroom boys who simultaneously sent to The Sun a list of all the judges who have been "soft on criminals". No criminal has ever been as shameless as this Government in providing a false alibi.

Yet Tony Blair was right when he accused the Conservatives of having a criminal record. It was the party of law and order that, when in Government, passed legislation making it compulsory to release prisoners after half their sentence, if the original tariff was four years or less. And even for serious crimes that carry a sentence of over four years, the Tories made release mandatory after two-thirds of the sentence.

It's crucial to realise that these stipulations allow the parole board no discretion: they are executive orders which make no reference to the conduct of the criminal while in prison. So when the former prison governor David Wilson told the BBC yesterday that the current system was an essential incentive to good behaviour by prisoners, he was speaking as if early release was still something that always needed to be earned.

When Mr Wilson went on to warn of the danger of riots if prisoners were obliged to serve the sentences originally passed, he was faithfully echoing the original reason why the Home Office introduced compulsory early release during the first Thatcher administration: I remember being told by a member of that Home Office team that it was fear of riots in overcrowded prisons that lay behind the change.

It's important to understand what this does and doesn't mean. It does not mean sentences have been reduced because that will help to rehabilitate prisoners. It does not mean sentences have been reduced because that will be a benefit to the public at large. It does mean the Home Office is so intimidated by the violent criminals in its own charge that it wants the public to absorb the risks it is not prepared to take.

For a long time I used to ask Home Office ministers whether the original sentences judges impose on convicted criminals are influenced by such considerations. No, no, I was always told: judges decide solely on the requirements of deterrence and public protection. Recently I discovered just how untrue this is. A QC friend sent me a copy of the Compendium of the Sentencing Guidelines Council. The very first guideline reads as follows: "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 and the punishment and deterrence of the offender." That guideline, by the way, dates from 1980: it is the oldest one in the book.

The Home Office has for years tried to prove that prisoners released early are less likely to reoffend than those who have served a full sentence; but the actual figures suggest a negligible difference in the reoffending rates. More pertinently, the public are the victims of hundreds of thousands of crimes - including rape and murder - committed by prisoners released early; crimes that could not have occurred if the original sentence had been observed.

Penal reformers argue that it is cruel to keep men in jail for any longer than is necessary. The question is: necessary for whom? It is absolutely true that prisoners have rights that must be respected: they should never be subjected to degrading treatment. But the public's right to safety makes its own powerful demands on our collective conscience. So it is good that Liberty is mounting a prosecution under the Human Rights Act against the Government on behalf of the mother of Naomi Byrne, a 30-year-old woman throttled and stabbed by Anthony Nelson, who was released by the parole board despite a horrifying record of sexual assaults and a psychiatric report that said that he was overwhelmingly likely to reoffend.

The Nelson case has already achieved notoriety. But two days ago I met someone whose only son was killed by a habitual criminal just two weeks after the man's mandatory early release from prison; he told me that his wife "welcomes death daily", so devastated is she by the murder of her boy. You will probably never read about this family; they don't want any publicity - but you should try to imagine how you might feel in similar circumstances.

It is frequently said that we already have the highest prison population per capita in Europe, as if that proved that we shouldn't build more jails. The more pertinent statistic, which does not get such a regular airing, is that per 1,000 crimes committed we have a dramatically lower rate of imprisonment than, for example, Ireland, Portugal, Spain and Greece. It's true that there are many people in our jails who should not be there, the wreckage of that other scandalous policy of the 1980s, "care in the community"; but the fact remains that the overwhelming majority of the recent increase in the prison population are men jailed for "violence against the person".

David Cameron has said that the Conservative Party, if elected, would reverse the policy it originated and stop all automatic early releases. According to the think tank Civitas, that would involve an increase of over 50 per cent in the prison population - and therefore a proportionate increase in the number of prisons. Will Mr Cameron commit the Conservatives to expand the capacity of our prisons on this scale? If he doesn't, then his claim to be "tough on crime" will turn out to be as detumescent as Mr Blair's.

d.lawson@independent.co.uk