Researchers say that one in three adults in England will be obese by the time London hosts the 2012 Olympics.
Almost 13 million men and women will be obese by 2012 based on current projections, experts from the Department of Epidemiology and Public Health at University College London said.
They based their projections on data from health surveys published between 1993 and 2004.
During this period, the proportion of obese people rose "significantly", from almost 13.6 percent to 24 percent among men and from almost 17 percent to 24.4 percent among women.
If the trend continues then 32 percent of men and 31 percent of women will be obese by 2012, they added.
The highest incidence of obesity will be in manual workers, with 43 percent of that group expected to be carrying excess weight but the projected figure was not much better for men and women working in non-manual jobs at 35 percent.
The researchers, writing in the Journal of Epidemiology and Community Health, said: "If recent trends in obesity continue, about a third of all adults (almost 13 million individuals) would be obese by 2012.
"These increases will affect lower social classes to a larger extent," they added.
"This highlights the need for public health action to halt or reverse current trends and narrow social class inequalities in health."
Separate research published Thursday showed that a quarter of four to five-year-olds in England's primary schools are already overweight or obese.
The Department of Health said Britain was taking the lead in the global fight against obesity.
"Obesity is the biggest health challenge we face - every year 9,000 people die prematurely," a spokesman said.
"The UK is leading the world when it comes to facing up to the problem and tackling obesity."
The government recently launched Change4Life, a campaign which aims to improve children's diets and activity levels.