It is not hard to understand why Alan Johnson, the Health Secretary, has decided that parents of every five-year-old found to be obese at primary school should be notified formally by letter. Britain may or may not be “in the grip of an obesity epidemic”, as the alarmists would have it, but Britons are certainly getting fatter. Obesity among children aged 2 to 10, in particular, has risen by nearly seven per cent in the past decade. Eighty per cent of obese ten to 14-year-olds go on to be overweight as adults. Adult obesity carries greatly heightened risks of stroke, diabetes and heart disease. The cost of NHS treatment for such conditions is borne by every taxpayer, and campaigners have spent the past week taunting the Government for letting its target date for cutting childhood obesity slip by ten years.

What better sign of a decisive stand than official warning letters to every parent involved? Mr Johnson clearly found the case compelling. Yet this initiative is wrong both in principle and as a matter of policy.

Hitherto, the argument most often made against schools formally notifying parents of obese children has been the risk of stigmatising those already struggling with their weight. This argument is weak. It may be that weighing all primary school pupils makes weight a topic of greater interest for them than it would otherwise be, but the National Child Measurement Programme that yields this data will be gathering it anyway, and the letters to parents will be private. Overweight children will be no more or less stigmatised than they already are.

The problem with writing to parents in this way is more fundamental. It involves using schools as a contact point with parents for an ever-larger, more invasive Government that has little inherent understanding of its own limits and has too often mistaken gesture politics for real reform. If parents show a taste for this sort of communication, they can be sure there will be more of it – starting with letters telling them their children are too thin.

Schools exist to provide education. That includes physical education, education on a proper diet and discussion on how humans as hunter-gatherers can coexist healthily with the technologies of convenience. Contact between schools and parents is, of course, of paramount importance, and there is no reason why a teacher who judges that a pupil’s weight is affecting his or her performance or wellbeing should not mention it in a letter home. But letters from ministries to parents via schools are entirely different things, and are no substitute for the timetabled sports and better meals those schools should be offering.

There is no room for complacency over childhood obesity – nor for hyperbole. Ed Balls, the Children’s Minister, only undermined his case last month when he overstated by 100 per cent the proportion of children who would be obese by 2050 if current trends continue. It is wrong also for campaigners to claim that children are at the mercy of the junk-food industry. The new rules banning junk food from schools are welcome, and the choice of healthy foods offered in their place has never been wider. The only reliable cure for obesity is for the overweight to make informed choices and eat less. Let schools stick to helping people to make those choices.