Eight out of 10 men and nearly seven out of 10 women in England will be too fat by 2020, according to new data released Wednesday.
Researchers said that while recent research showed obesity among children levelling off, instances among adults show no sign of doing the same.
Some 41 percent of men aged 20 to 65 will be obese by 2020, with 40 percent overweight, according to the figures from the National Heart Forum, based on data from the Health Survey for England. That makes a total of 81 percent.
Among women, 36 percent will be obese and 32 percent overweight - a total of 68 percent.
By 2050, this will lead to sharp increases in the number of people suffering strokes, high blood pressure, heart disease and diabetes, researchers said.
"These trends demonstrate that the cautiously optimistic picture we presented in November 2009 for a levelling off of future obesity rates among children is not mirrored in adults," said Professor Klim McPherson of Oxford University, who also chairs the National Heart Forum.
"There are already more men who are obese than who are of a healthy weight and by the end of the decade, obese men and women could out-number those who are overweight."
Britons were "being overwhelmed by the effects of today's 'obesogenic' environment, with its abundance of energy-dense food and sedentary lifestyles," he added.
The research used figures from 1993 to 2007 to predict future obesity levels in England.
Obesity is defined as a body mass index (BMI) of over 30, overweight as between 25 and 30 and healthy between 20 and 25. BMI is a commonly used indicator based on a person's height and weight.