The Prime Minister has let down both himself and his Lord Chancellor

Is the public really so unable to see the difference between crimes of violence, or intended violence, and unaggravated burglary?

Lord Irvine of Lairg, the Lord Chancellor, is frequently his own worst enemy. The choice in wallpaper, the absence of humility about the fact that he has never been elected, the epicurean tastes, all help to make him, of all the members of the Cabinet, the one most custom-built for public obloquy. The post itself is strangely archaic, mixing judicial and political roles with an abandon that no written constitution would allow. You might have thought that in an era when plenty of intelligent people think that it should be swiftly replaced by a far more conventional Minister of Justice, the present incumbent might show a rather more uncontroversial and reticent profile to the world. But Alexander Irvine simply isn't made that way.

What's more, to judge by Tony Blair's performance in the House of Commons yesterday, Lord Irvine's similarity to Falstaff is more than a matter of mere demeanour. Maybe Mr Blair is not, despite his long, close and convivial relationship with Irvine, precisely Prince Hal to his Sir John. And no, he didn't quite say, "I know thee not, old man." Nevertheless, there was just the faintest echo at Prime Minister's Questions yesterday of that chilling moment in Henry IV Part 2 when Hal rejects his old friend and companion.

For if this was a defence of Lord Irvine's assertion that burglars, particularly those at the outset of a career in crime, should not be sent to prison if at all possible, it's frightening to imagine what an attack on it would be like. Faced with the choice between coming resoundingly to the aid of his old friend or demonstrating that he was not a jot softer on crime than Iain Duncan Smith, and the tabloids from which the Conservative leader had taken his cue, Mr Blair, with a notable lack of sentimentality, unhesitatingly chose the second course.

What Lord Irvine had been doing, of course, was to back Lord Woolf, one of the most eminent judges to hold the post of Lord Chief Justice, on new sentencing guidelines which urged the courts to consider alternatives to jail for novice burglars. The Prime Minister correctly pointed out that a more careful reading of the guidelines demonstrates that it is nothing like the thieves' charter its many enraged critics have depicted it as. And he was careful not to disavow the notion underlining the Woolf guidelines that for some first-time offenders drug treatment and a rigorously run community service order was a more appropriate sentence.

But politics is also about tone. And the thrust of Mr Blair's riposte across the dispatch box to Mr Duncan Smith (bringing Question Time forward to the morning has done little to ensure that the event generates more light and less heat) was that the Labour government had been more assiduous in sending first-time offenders to prison than its Conservative predecessors.

None of this would matter very much if it were not for the fact that Lord Irvine was right. The last relevant Home Office research – which, inexplicably, is some five years old – shows that community punishment orders had the effect of reducing reconviction rates by 4.9 per cent while prison sentences showed only 3.6 per cent. Sure, statistics on the subject are sparser than they should be and by no means consistent. But even those most hawkishly attached to the idea that prison works have been unable to come up with evidence that custodial sentences are more effective at preventing recidivism than community punishment. Not to mention that community sentences are much cheaper to run than the £37,000 a year it costs to keep an offender in prison – or more important still that the country is facing a universally acknowledged problem of prison overcrowding.

The policy people in Downing Street (who weren't forewarned about the exact nature of Irvine's remarks, something which always sends them into a paddy) sucked their teeth audibly. Partly they believed Irvine shouldn't have referred to prison overcrowding if he was trying to make the case that community punishment worked because it suggested a mixture of motives. That is surely doubtful. It implies that the public aren't grown-up enough to be told that prisons that are overcrowded are even less capable of educating or rehabilitating offenders than ones that aren't. The defenders of Lords Irvine and Justice Woolf – and there haven't been many of them – have been depicted as out of touch idiots who think burglary doesn't matter or cause deep grief. What works should count for much more than the simple desire for retribution which informs many of the attacks on the new guidelines.

Irvine also ran into trouble because he suggested that alternatives to prison should be the starting point even for some second-time burglars. But that too is just what the Lord Chief Justice said – in cases where the stolen goods were of little value and there was no damage to property.

Which is what is so disappointing about Mr Blair's response yesterday. He is fully capable, and with a huge parliamentary majority has the ideal opportunity, to do what Margaret Thatcher, from a wholly different ideological position, frequently did so effectively – be a teacher politician. Instead of making the weather by saying some of the above yesterday, he laid himself open to the charge of following rather than leading public opinion on the issue.

The defence, of course, is that he was left in a highly uncomfortable position by the apparent contradiction between the Irvine line on burglary and the Blunkett line of minimum five-year jail sentences for possession of firearms. Certainly Irvine and Blunkett should do more to overcome their frequent differences and sound more consistent on an issue on which the Government has long been woefully inconsistent. There was quite a lot in Mr Duncan Smith's charge yesterday that the quantity of government initiatives on crime is a lot more impressive than their quality. But once again, is the public really so unable to see the difference between crimes of violence, or intended violence, and unaggravated burglary?

Lord Irvine may be an unlikely hero. He can be spectacularly wrong – as he was in the same interview when he sought to deflect the groundswell for further Lords reform by suggesting that the Upper House should be either fully elected or fully unelected. If that's the case, why did a White Paper bearing his own name suggest a hybrid system last year?

But that doesn't mean that he was wrong in this case. There were complaints from his colleagues that he was acting as a lawyer and not as a politician. Which of course is just what used to be said about his Tory predecessor, Lord Mackay, when he produced some humane or sensible proposal that offended hardliners. But politicians are not the only repositories of wisdom in this area. Irvine was right, as well as performing a historic duty, when he defended the Lord Chief Justice. And Tony Blair was doing a disservice to more than his long friendship with the Lord Chancellor when he left him swinging in the breeze yesterday.

d.macintyre@independent.co.uk