f Jamie Oliver can stop getting into ding-dongs with Nora the dinner lady, he may still be able to do something to turn around the dismal - and all too representative - over-processed junk being fed to children. Delivering such an embarrassing public kick up the backside to the complacent powers that be, who have allowed school meals to degenerate into such a disgraceful fiasco, is a laudable mission.

Mr Oliver recently visited schools in a South African township and pointed out on Radio 4‘s Food Programme that even the poorest of the poor there were eating a more nutritious lunch than Britain’s children.

He is clearly deadly serious about highlighting the current school meals mess. More power to his elbow. But was his programme on Channel 4 first and foremost about improving school meals, or a gentle probe into Mr Oliver’s psyche which reveals the stresses of being a celebrity chef.

In that latter respect, a question needs to be asked. Can anyone who makes a lot of money from promoting a supermarket chain really be taken seriously when they turn their attention to improving Britain’s eating habits?

It is all very well for Sharon Osbourne to do the job for Asda. Pop stars run out of cash at regular intervals. And Prunella Scales is a seasoned actress for whom promoting Tesco is just another character role. But when Jamie Oliver makes adverts for Sainsbury he brings an aura of gastronomy and food quality to an industry that simply does not merit it.

One of the main reasons children eat so badly is because supermarket shelves are laden with over-processed, unhealthy food. Changing school meals for the better would mean that children had a model for sane, balanced, nutritious eating.

But what about all the rubbish in the kitchen back home, 80 per cent of which is supplied by supermarkets? Next time you are in one, take a look at what products are on prominent special offers, such as “buy one get one free” and multibuy deals.

The vast majority involve everything the nation, and particularly the nation’s youth, would be better off not eating, all those items that children scoff by the barrel-load, the same stuff that needs to be eliminated from school meals. We are talking bumper bags of flavoured crisps, gummy sweets lurid with chemical colourings, fizzy drinks that rot teeth and bones, pseudo-nutritious breakfast cereals and squashes loaded with sugar, and sweets and biscuits stiff with artery-clogging, chemically hardened fat.

The supermarkets like to affirm paternalistically that they do their bit to improve Britain’s health. If we are turning into a nation of hypertensive fatties, it is nothing whatsoever to do with them.

When worrying trends about the UK’s health surface, our large retailers, even though they supply the bulk of what ends up on our plates, rush to distance themselves from any culpability.

Thanks to the supermarkets’ self-serving myth that people do not have time to cook any longer - surely the antithesis of the Jamie Oliver philosophy - parents are not loading up their trolleys with good, wholesome raw ingredients destined to be cooked from scratch, such as that pricey Jamie Oliver Taste the Difference beef, but all those convenient pies and ready meals that have turned up, somewhat alarmingly, on the lists of product recalls following the discovery of Sudan 1, the carcinogenic dye that is illegal in food.

Oh, and by the way, once the Sudan 1 brouhaha has died down, look out for bargain “two for one” offers on all affected lines to restore sales to pre-Sudan 1 levels.

The business logic is faultless. Supermarkets make more money out of selling value-added processed junk than they do selling good food. There is a limit to how much they can charge for a potato - even well-scrubbed, heirloom varieties. But process a nondescript white spud into smiling potato shapes and the sky’s the limit.

Perhaps Jamie Oliver is just operating a Robin Hood policy, taking the supermarket shilling to underwrite his school meals campaign. But let us be clear here: there is a conflict.

Joanna Blythman’s latest book, Shopped - The Shocking Power of British Supermarkets, has just been published in paperback by Harper Perennial (Pounds 7.99).