A government report claims that if the population continues to gain weight at the same rate, the majority of Britons will be obese by 2050.
By then scientists predict that 60% of men, 50% of women and a quarter of all children in the UK are likely to be clinically obese.
And as a result they would be in a weight category associated with serious health problems, such as type 2 diabetes and heart disease.
Only 10% of men and 15% of women were expected to have a "healthy" weight in relation to their height.
The consequent impact of chronic health problems on society would cost Britain more than Â£45bn a year, the report claims.
The predictions are contained in the latest report from the government's Foresight think-tank, which looks at future trends.
Almost 250 experts took part in the two-year investigation which culminated in the report Tackling Obesities: Future Choices.
The key conclusion was that human biology is simply out of step with modern life in the 21st century, with its labour-saving devices, motorised transport, sedentary work and cheap high-energy foods.
Weight gain was said to be a "default" condition that inevitably resulted from the environment and culture people lived in.
Sir David King, the government's chief scientific adviser and head of the Foresight programme, 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."
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