Today, two thirds of all adults and one third of all children are either overweight or obese. By 2050, on current trends these figures will rise to almost nine in ten adults and two thirds of all children. By then, obesity, which is already responsible for 9,000 premature deaths each year, 58% of all type 2 diabetes, 21% of heart disease and a nine-year reduction in life expectancy, will lead to a seven-fold increase in direct health costs with wider costs to society of around £50bn.
The increase in the number of children who are obese is a particular cause for concern. Obesity is not something that children tend to grow out of. Fifty-five per cent of 6-9 year olds and 79% of 10-14 year olds who are obese will remains so into adulthood. There is a very real danger that significant numbers of today's children will live shorter lives than their parents and spend more of their years in poor health.

Obesity is the product of a simple imbalance between energy intake and energy expenditure. Because it is a lifestyle disease, it requires us to change the way we live. Despite the abundance of information about how to protect our health, the straightforward advice about what to eat, how often to exercise, how much we should drink, is easy to ignore and frequently lost in a cacophony of conflicting messages.

While most would be shocked by a newspaper story about a nine-year-old who weighs 14 stone, the distinction between being a bit overweight and obese is not always so obvious. Research shows that only 17% of parents with an obese child identified that child as having a serious weight problem, and the majority of parents overestimate the amount of exercise their child engages in and underestimate the amount of food they eat.

So how should a serious political party of the 21st century faced with these acute and growing problems react?

The scientists involved in the Foresight report, commissioned by the government, highlighted the fact that for an increasing number of people, weight gain is inevitable and largely involuntary as a consequence of exposure to a modern lifestyle. They used the term "passive obesity", and pointed out that it particularly effects the socially and economically disadvantaged.

Our strategy made clear that in approaching this problem, we reject both the "nanny state," which polices shopping trolleys and institutes exercise regimes and the neglectful state, which wipes its hands of the problem, and wags the finger in the direction of the most vulnerable families in the vague hope that they will do as they are told.

The Conservative party has apparently chosen this approach. Reading David Cameron's Glasgow speech, I was struck not by how much the Tories have changed, but by how little. It attracted predictable support in the pages of the Spectator, where, in an article headed "Shouting abuse at fat people is not just fun, it's socially useful," Rod Liddle congratulates Cameron for "telling these awful people it's all their own fault that they are hideous, poor and stupid."

It's easy for politicians to stand on the sidelines accusing the impoverished, the fat and the excluded of only having themselves to blame. But before we evoke the Victorian notion of the deserving and undeserving poor, we should take a moment to consider how complex these issues really are.

Just as the government has a moral duty to tackle poverty and exclusion, so it also has a duty to address obesity. This is not a licence to hector and lecture people on how they should spend their lives – not least because that approach simply won't work. The state cannot and should not micromanage the choices that people make in their daily lives. We are calling on everyone – from the smallest community keep fit class to the biggest retailers in the land – to join in this campaign to change the way we live our lives.

It is simply wrong to suggest that the only solution to deep-rooted problems such as obesity is for people to be more responsible. Research shows us that vilifying the extremely fat doesn't make people change their behaviour. Commentators who point and shout at pictures of the morbidly obese simply fuel the problem. Those whose seriously unhealthy lifestyles are not advertised by their waste lines will simply say: "Well, that's not me. I don't need to change what I do." But if you present the message more intelligently – if you explain to parents that many children, regardless of their size, have dangerous levels of fat in their arteries or around their organs, and this may reduce their life expectancy by up to 11 years – then people respond.

This is an edited version of a speech delivered by Alan Johnson to the Fabian Society. To read the full text, please go here.