I have every sympathy with David Walker, the Lanarkshire GP who proposed a tax on chocolate as a way to ease the obesity crisis and reduce the spread of type 2 diabetes, only to see it narrowly voted down at the British Medical Association's conference in Clydebank on Thursday.

Like Dr Walker, I was appalled to learn that fatties across the nation are consuming their full daily recommended intake of calories in the form of sweeties alone, over and above the mounds of cack they swill down at the times of day the rest of us call “meal times”, but which for an increasing proportion of Britons merely represent thrice-daily upward spikes in the ceaseless, rolling gobble that constitutes their lives.

Like him, I am horrified by how much it costs the NHS to look after so many people who have made themselves ill simply through a lack of willpower and/or self-respect. It's not just type 2 that is in danger of crippling the health service. It is joint replacements, gastric banding, heart disease, other organ failures and all the respiratory problems that follow in obesity's gigantic, swaggering wake. The total annual cost of human fatness to the NHS is estimated at £1 billion a year. One billion pounds! That's an awful lot of Toblerones.

Then there's the cost to the economy in sick pay and incapacity benefits arising from obesity: a staggering £2.5 billion per year. And, of course, the extra transport costs linked to obesity - because the fatter we grow, the fewer people can fit in a bus or train, and the more fuel is required to move each human - that run to £250 million annually.

So it comes to something like £4 billion a year that fat people are costing our economy, not to mention the immense personal harm they are doing to themselves. Over the past ten years Britons have, on average, put on 6lb each. Two thirds of us are overweight (that's you, fatty!), 10 per cent of premature deaths in this country are fatness-related and by next year obesity will take over from smoking as Britain's biggest preventable killer. And it has got to the point where I, like Dr Walker, have lost faith in humanity's ability to save itself.

Like Dr Walker I believe that the answer lies in taxation. You have to hit people where it hurts most, which in the case of fat people is in their wallets, because literally hitting them (fun though it is) doesn't hurt them at all, what with their being so fat.

But where I differ from Dr Walker is that I do not believe in taxing chocolate. First of all, the money thus raised will not get anywhere near to recouping the £4 billion a year we are owed by the gut-buckets; and second, it would constitute an infringement of the rights of normally sized people to eat chocolate.

Personally, I love a few squares of posh chocolate with a cup of tea. I'll guzzle a Snickers with the best of them if I've exhausted myself playing football, or simply chasing fat people across Hampstead Heath with a stick. But why should I - with my healthy weight of twelve and a half stone, perfect body mass index (BMI) of 24 and damn near supernatural total body fat percentage of 12.5 - have to pay more for it? And if the choccy tax were extended to high-fat fast foods, as has been mooted, that again would penalise me. Sometimes I need a Big Mac to keep me going, or an entire KFC bargain bucket. But they can't hurt me because I'm not fat. And never will be.

Calories are not like cigarettes, alcohol and guns - they are not, in themselves, bad for you. To tax them is to punish us all for the sins of the porkers. Keeping the whole class in after school because Fatty couldn't control himself.

Taxing the things that contain the calories is cumbersome, unworkable and fiscally inadequate. What we have to do is tax fat people directly. I admit that this is a form of head tax (or rather, in this case, an arse tax) of the kind that has led to all sorts of social unrest in the past, but don't worry: fat people are far too lazy to riot.

The key here is to make people pay more income tax the fatter they are, so as to incentivise weight loss and repair the damage done to the economy. And the obvious way to create a tax that is both income and blubber-related is to take each individual's annual tax liability (as derived by the current system) and then multiply it by the square root of his BMI over a hundred. So, where L is the normal tax liability:

This way, for example, if your BMI is 36, which is halfway between obese and morbidly obese, you would pay 6 per cent more tax than a normal person. If you currently pay £5,000, you will pay £5,300. It's not a lot in punitive terms but it will comfortably raise the sums we need, and it will give fat people an opportunity to choose between paying up or laying off the sweeties - which is only fair in a sophisticated democracy such as ours.

Had Dr Walker taken this far more evolved and fairer taxation plan to the BMA last week, I am sure that they would have approved it to a man.

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