Even the slowest learner will eventually get the message. Burn those calories in the pool and then fill up on junk. Contradictory or what? And yet this warped thinking seems to be the political zeitgeist, voiced by none other than Culture Secretary Tessa Jowell.

For years health campaigners have been calling for a ban on junk food advertising to children to protect their health. Lately these demands even seemed to have found a listening ear at the generally conservative, status quo-upholding Food Standards Agency. Ms Jowell, however, is sceptical about a ban, preferring to blame lack of exercise, not overeating, or bad eating, for the growth in obesity.

It is a message designed to play well with a food industry which was beginning to get a little twitchy that the Government might clamp down on junk food.

No one can argue with exhortations to take more physical exercise. But what does it cost the Government to say this? No industry body is going to get hot under the collar about such a prescription and the buck has been passed by government on to individuals. Avoiding obesity becomes the personal responsibility of citizens, with teachers told airily to pass on the good word in a legislative vacuum where virtually nothing has been done to create a climate in which those health messages might take root. The minute pupils walk out of the classroom they are delivered into the welcoming arms of junk food barons: in the school dining hall, at the tuck shop and vending machine, at the shops outside the gates.

It is time that we ditched the crude “calories in must equal calories out” approach, also known as “Eat as much junk as you want as long as you burn it off” because it is a totally ineffective tool for tackling the causes of obesity. An avocado, for example, is a high-fat food but it is a nutritional treasure, laden with health-promoting vitamin E. There are any number of spuriously healthy “diet” drinks and snacks trumpeting low calories, laden with chemical additives. But only the very naive or the very devious can advance the argument that these positively contribute to a healthy diet.

Increased physical exercise, though unquestionably a good thing, is rapidly shaping up to be the excuse government gives itself for standing aside while purveyors of junk have a profit-generating free for all at the expense of children’s health, and therefore, the future health of the nation. Telling kids to go and kick a ball around the pitch is no substitute for taking spiralling obesity in hand.

What might be done to improve eating habits? Earlier this month on BBC Breakfast News, chef Garry Rhodes was totally clear on the point: make cooking compulsory in schools. Like all top chefs, Mr Rhodes can see that a country that no longer cooks from scratch from raw materials is likely to live on over-processed, nutritionally-debased junk.

Britain’s diet time bomb is ticking away and requires radical intervention, the equivalent of a World War Two Dig For Victory campaign. This should involve a ban on junk food advertising to children and the introduction of a universal entitlement to free, limited choice, school meals, pegged to sound nutritional standards. Such public health measures are already in place in forward-thinking, healthy countries such as Finland and Sweden and a free school meals Bill will soon be put before the Scottish Parliament.

Without such action, Tessa Jowell can exhort us to work up a sweat until she is blue in the face. It won’t make a blind bit of difference.

Joanna Blythman’s new book Shopped - The Shocking Power of British Supermarkets, 4th Estate, pound;12.99 will be published on May 5