The murder on Saturday of 18-year-old Robert Knox has prompted, as have the other 27 teenage murders so far this year, a flood of suggestions as to how we can deal with the epidemic of knife crime that seems to have infected our streets. From analysis of the role of parents to depictions of the gang culture and turf wars that blight so many areas, most have added something useful to our understanding.

So it might seem that another comment is hardly needed. Yet for all the analysis that has been offered and the policy ideas that have been suggested, one basic point seems to have been forgotten. We have yet to try properly using the laws already on the statute book, let alone start properly punishing those found in possession of knives.

Over the past decade, the number of convictions for carrying a knife has risen from 3,360 in 1997 to 6,314 in 2006. Of those convicted in 1997, 482 were teenagers, rising in 2006 to 1,256. That near trebling in the number of teenagers convicted is bad enough. Worse, however, are surveys showing that about one in five teenagers say that they carry a knife with them.

Given the rapid development of a teenage culture in which carrying a knife is seen as normal, not to say essential for self-defence, it is understandable that there have been calls to toughen the relevant laws. The current maximum sentence for knife carrying is two years, or four years if the knife is carried to school.

But since we do not enforce the existing laws properly, it is fatuous to suggest that tougher maximum penalties would serve any useful purpose. They would be ignored just like the existing maximum penalties.

In 2006, only nine of the 6,314 people convicted of carrying a knife were handed down a maximum sentence. Most were given a caution. And I would bet a small fortune on not one of those nine criminals - 0.14 per cent of those convicted - actually being made to serve the full sentence they were given.

Despite the penalties available, the authorities treat this potentially deadly crime as an infringement of the law akin to pilfering an apple from a grocer. This has to change. The courts must use the punishments available to them. Children need to understand that, if caught, their childhood will effectively be over and they will suffer severe punishment.

That also means that the police must be given full powers to stop and search children. But instead, not only do the courts and CPS treat children found with knives with kid gloves, dangerous idiots such as Sir Al Aynsley-Green, to whom we pay £130,000 a year for his wisdom as the Children's Commissioner for England, warn that allowing police the power to search children might antagonise them. That just about sums up how the whole edifice works: God forbid that a potential murderer is upset by having his coat examined.

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