IN HIS report on National Health Service spending just published, Sir Derek Wanless warns that Britain's obesity epidemic is spiralling out of control. On current trends, he says, 33% of men, 28% of women and 20% of children will be obese (in other words, extremely fat) by 2010. Now it's official: far from slowing the incidence of obesity as it pledged to do, Labour has presided over a dramatic rise, one that threatens to bankrupt the NHS. No health service, however effective or well-funded, can ever cope with record numbers of ailing citizens suffering the consequences of obesity; conditions like diabetes, heart disease and stroke.

I had my own personal encounter with Britain's sprawling fat problem earlier this week while travelling back from holiday. I found myself on a full flight, sat next to an extremely fat woman whose body size exceeded the dimensions of her seat. "Would you mind if we left the arm rest up ?" she asked, explaining that she was "rather broad around the hips", an understatement of epic proportions.

Initially, I gave her the answer she wanted to hear, then hastily backtracked when I realised that this would mean enduring an eight-hour flight in an already cramped space with half of her ample frame occupying my seat. As it was, even trying to put the arm rest down proved impossible because it was constantly forced upwards by her bulging thigh. I felt sorry for the woman, certainly. She must have had a desperately uncomfortable journey. But I also felt put upon. How much longer before territorial disputes over plane seats become as commonplace as neighbour fights over Leylandii hedges ?

Travelling through airports offers a graphic demonstration of how we have totally lost the plot with obesity. Last year, in Beauvais airport outside Paris, I observed three check-in queues. The first two, bound for Stockholm and Amsterdam, consisted of people of a generally slim or at least normal build, a bit like British people would have looked like in the 1960s, say. The third queue, bound for Prestwick, was a national embarrassment. Almost everyone was overweight. They looked out of puff, self-conscious and robbed of any vitality they might otherwise have had by the burden of dragging around all those excess kilos. In denial, too, trying to squash themselves into the those jumbo-sized jeans now designed to accommodate sizes 18-32. How the prospect of returning home - where they might blend in with the rest of the plump population - must have appealed!

This ever-expanding national girth is a reflection of our over-tolerant attitude to fatness. It is a much more comfortable social experience to be fat in the UK or the US than it is in Europe. Call it body fascism if you like, but in Europe excessive weight gain, much like excessive drinking, is socially disapproved of.

In the UK, on the other hand, being fat is so common as to be acceptable, even expected. Indeed we live in the era of fat militancy, borrowed from the US, where clothes shops catering for teenagers are taken to task for not stocking previously unheard-of sizes upwards of extra large, and where the design of everything from toilet seats to bus aisles is rapidly being scaled up to cater for greatly enlarged dimensions.

Many behaviour patterns in Britain add up to what epidemiologists now refer to as an "obesogenic" environment. Workaholism leaves us exhausted, craving recovery time rather than active leisure time. We believe that we don't have time to cook, so we live on a diet of gut-expanding processed food. Stranger Danger fears keep children imprisoned at home, glued to Game Boys and telly, not outside playing with a ball. In Scotland, a peculiarly Celtic fatalism allows us to absolve ourselves of personal responsibility for our wellbeing and look passively to the NHS to undo any damage that we may do to ourselves.

In our defence, we have been miserably let down by government. Since Labour came to power it has been in the pocket of the food industry and its advertisers, and terrified of being criticised as a nanny state if it is too prescriptive in telling people what to eat. Even with the kick up the backside delivered by the Wanless report, it would be naive to wait for the government to weigh in with public measures to save us from ourselves.

So what is going to give, other than our straining seams, if the population keeps ballooning? Even further crippling rises in national insurance contributions will be insufficient to deal with the strain on the NHS. That will leave society having to make very tough decisions. Should children so fat that their health is hopelessly compromised be taken into care to save them from "neglectful" parents who raise them on cola and takeaway? Social services have already taken three children into care on these grounds. Will we have to ration medical treatment to fat people on the grounds that their illnesses are self-induced ?

I don't mean to be entirely unsympathetic. A few people become fat for complex medical or psychological reasons outwith their control. But most people are fat just because they can't be bothered to take themselves in hand, and their lives are the poorer for it. They need to get a grip.