Obesity does not result simply from over-eating and a lack of exercise but is a consequence of modern life, a think-tank said.

Being overweight is a far more passive phenomenon than is often assumed, according to Foresight.

It found in a report that the technological revolution of the 20th century has led to weight gain becoming unavoidable for most people because our bodies and biological make-up are out of step with our surroundings.

'Stocking up on food was key to survival in prehistoric times, but now with energy-dense, cheap foods, labour-saving devices, motorised transport and sedentary work, obesity is rapidly becoming a consequence of modern life,' said Sir David King, the government's chief scientific adviser and head of the Foresight programme.

The report, sponsored by the Department of Health, is the result of a two-year study into the causes of obesity involving almost 250 experts and scientists.

They predicted that the so-called obesity 'epidemic' would take at least 30 years to reverse.

Until now, the government has concentrated on encouraging people, particularly children, to lead a healthier lifestyle, eating less fattening foods and taking more exercise.

But King said a wholesale change in attitudes was needed.

'Foresight has, for the first time, drawn together complex evidence to show that we must fight the notion that the current obesity epidemic arises from individual over-indulgence or laziness alone,' he said.

'Personal responsibility is important, but our study shows the problem is much more complicated. It is a wake-up call for the nation, showing that only change across many elements of our society will help us tackle obesity.'

The researchers said there was no single 'magic bullet' solution; even a new appetite-suppressing drug would not be the answer, because the problem is systemic.

Tackling obesity, like tackling climate change, requires a range of changes in society, from increasing everyday activity through urban design and transport systems to shifting the drivers of the food chain and consumer purchasing patterns to favour healthier options.

If current obesity growth rates continue, some 60 per cent of men, 50 per cent of women and 25 per cent of children in the country will be obese by 2050, according to the researchers.

Associated chronic health problems are projected to cost society an additional 45.5 billion pounds per year.

Health Secretary Alan Johnson told parliament that government alone could not tackle all the problems.

'The chilling reality is that modern life makes us overweight,' he said. 'In a sense, we are the victims of our economic success. Tackling this problem calls for a fundamental shift in approach.'

He cited a number of measures that had been taken and suggested the food industry could do more.

There was encouraging evidence for example about the 'traffic light' package labelling system and he would try to work with the industry to see if it could be adopted across the board.

Public health minister Dawn Primarolo said there had been progress with more physical activity at school, healthier school food for children, clearer food labelling and a ban on TV ads for junk food aimed at children.

She said tackling childhood obesity remains a 'key cross-government priority', with the aim to cut the proportion of overweight children to 2000 levels by 2020. Reuters