LONDON (Reuters) - Obesity does not result simply from over-eating and a lack of exercise, but is a consequence of modern life, a government think-tank said on Wednesday.

Weight gain does not result from people's actions -- such as over-indulgence or laziness -- alone, and is a far more passive phenomenon than is often assumed, according to Foresight.

It found that the technological revolution of the 20th century has led to weight gain becoming unavoidable for the majority of the population, 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 Department of Health-sponsored project is the result of a two-year-long 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.

The government has, up until now, focused policy designed to tackle obesity on encouraging people, particularly children, to lead a healthier lifestyle, eating less fattening foods and taking more exercise.

But Sir David said a wholesale change in attitudes towards obesity is required to address the problem.

"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 the design of the built environment 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 percent of men, 50 percent of women and 25 percent 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.

Commenting on the report, public health minister Dawn Primarolo said: "We have made progress with improved physical activity levels at school, healthier school food for children, clearer food labelling and tougher restrictions on advertising foods high in fat and sugar to children -- but we know that we need to go further and faster."

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.