The patient in my surgery yesterday morning was only in his 40s, but he was leaning hard on a walking stick. He is morbidly obese, weighing nearly 20st, and has lost two toes to type 2 diabetes.

The grim task fell to me of warning him that he might now lose toes on the other foot. I fear he could eventually need a below-knee amputation. He has been trying to change his diet, without success. The food he is eating is quite literally making him ill, and could even kill him.

For a GP, that’s hard to see. But just as upsetting was the plight of two children, both seriously overweight, who were brought to my clinic by their obese mother last week. The children, a boy and a girl, are refusing to eat any of the foods I have recommended.

They won’t touch meat or vegetables: they want sugary cereals, burgers, pizza and ice cream. And between meals, they fill up on snacks laden with salt and sugar, such as crisps and biscuits.

You might think this mum is irresponsible. You could feel the middle-aged man is a fool to himself. You’d be wrong in both cases.

They want to change their lives, but they need help.

That’s why I fiercely welcome the national debate currently raging about a proposed levy on salty and sugary food, the so-called ‘snack tax’ being championed by the Government’s ‘food tsar’ Henry Dimbleby.

Such a measure cannot come too soon, to help save my patients — and the NHS.

The Dimbleby report has been met in some quarters by a barrage of criticism. An Old Etonian and scion of the Establishment (his father is the veteran broadcaster David Dimbleby), Henry made a fortune after co-founding the Leon chain of healthy-eating fast food restaurants.

He has been painted as a millionaire toff ordering the rest of us about and telling us what we should or shouldn’t eat.

But the criticisms are unfair. Like Jamie Oliver before him — who also campaigned for better food — Dimbleby has nothing to gain financially by speaking out. Concerningly, Boris Johnson last night appeared to distance himself from the new proposals, saying he did not want to land ‘hard-working people’ with ‘extra taxes’.

This is a serious — and potentially deadly — mistake. It is hard-working people — the poorest in our society — who suffer the most from the ill health brought about by bad diets.

Meanwhile, the beauty of Dimbleby’s proposals is two-fold, because a snack tax will generate funds to promote healthy eating, even as it discourages poor diet.

That means money will be available to provide subsidised vegetables to the poorest families, and to fund cookery lessons — a crucial plan if people are to wean themselves off processed food and takeaways.

A modest levy on the most unhealthy items could lead to people making much better choices, extending the length and quality of their lives and, over time, saving the health service billions.

In a perfect world — as with the levies on cigarettes — the tax would in fact raise no money at all as people turned away from the most unhealthy foods.

Healthy eating, for people on low incomes north of Liverpool where I work, is practically impossible.

The cheapest, most affordable foods, and the ones most heavily promoted by fast-food outlets and supermarkets, are all highly processed. Many of these junk foods are so low in nutrition, and so crammed with additives, salt, fat and sugar, that my patients end up at once overfed and undernourished.

For vulnerable groups, such foods are close to poison. They don’t taste like poison, of course. Once you have acquired a liking for artificial meals, such as pre-packaged sausage rolls, crisps and glazed doughnuts, they are dangerously addictive. But there is almost no goodness in them — and that is borne out in the statistics of health and obesity that dominate the national discussion.

Poor diets contribute to some 64,000 deaths per year in Britain, costing the economy a staggering £74 billion annually.

Almost two-thirds of British adults are now overweight, with 28 per cent being obese. Childhood obesity almost doubled from 8.5 to 13.3 per cent between 2007 and 2020 alone.

But it is life on the ground as an NHS doctor that shows you the true scale of the problem — and the terrible suffering it brings — close up.

The children of the young mother I mentioned earlier don’t see anything wrong with their weight or their diet.

And why would they? All the adults in their family are obese, and so are most of their friends and their friends’ parents.

Often, healthcare professionals and teachers shy away from talking about weight and nutrition, for fear of being accused of ‘fat-shaming’.

But if we fail to get the obesity crisis under control, it will overwhelm the NHS. In the area I work, diabetes has soared 800 per cent since I started out as a GP in the mid-1980s, a figure being replicated across the country.

The health service simply cannot cope with the avalanche of health problems this is bringing. Type 2 diabetes is a chronic condition that causes serious illness, often for decades.

And it is just one of many life-threatening sicknesses caused directly by poor diet. Heart disease is also rising steeply, as are several kinds of cancer.

Throughout my career, I’ve been frustrated by the false sense of security engendered by the existence of a really good health service. The NHS is among the best healthcare systems in the world, and it’s free at the point of use.

That’s astonishing — but there is a downside.

We’ve become accustomed to thinking that the NHS, because it is always available, will somehow always help us. However badly we break our bodies, the health service will patch us up.

That’s a fatal misconception. Doctors everywhere struggle to reverse the ravages of bad diet. They can’t just prescribe a few pills.

It is essential that the Prime Minister heeds Mr Dimbleby’s National Food Strategy and acts on its findings.

To fail to do so would be deeply irresponsible — and the effects could, in time, be dramatic, halting the inexorable rise in obesity in this country.

Best of all, I know it can work. I see the evidence of better nutrition improving my patients’ health in my surgery every day.

Once they understand how junk food affects their weight and health, many people are capable of changing — they just need support and for better, nutritious food to be affordable.

The Government has a duty to provide that health.

If it doesn’t come soon, I don’t know how much longer our health service will be able to cope.

Dr David Unwin is ambassador for the All Party Parliamentary Group on diabetes.