Eight out of 10 men and almost seven in 10 women will be overweight or obese by 2020, according to a study published today.
While data suggests childhood obesity may be levelling off, the picture for adults is "less optimistic".
Not only will people be fatter but the incidence of diabetes, stroke and heart disease will dramatically increase, the report added.
Led by Professor Klim McPherson from Oxford University, who is also chair of the National Heart Forum, the study updates predictions in a 2007 Foresight report, which was based on data from 1993 to 2004.
The latest research uses complete figures from 1993 to 2007 to predict future levels of obesity in England.
The study said: "Unlike the recent report on child obesity, which showed some indications of a plateauing or at least a significant reduction in the rate of obesity, the future projections for adults are less optimistic."
Some 41% of men aged 20 to 65 will be obese by 2020 and 40% will be overweight, the new predictions show.
Meanwhile, 36% of women will be obese and 32% will be overweight.
Among those aged 40 to 65, 44% of men and 38% of women will be obese while 40% of men and 32% of women will be overweight.
Obesity shows no sign of decreasing among adults and the incidence of associated illnesses - such as heart disease - will only rise, said the report.
By 2050, there will be a 23% rise in the prevalence of obesity-related stroke, a 34% rise in obesity-related high blood pressure, a 44% rise in obesity-related coronary heart disease and a 98% rise in obesity-related diabetes.
Prof McPherson 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.
"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.
"The serious health problems associated with obesity mean that these continuing rising trends will impose a substantially increased burden on the NHS.
"The Government needs to redouble its efforts to tackle obesity.
"We are being overwhelmed by the effects of today's 'obesogenic' environment, with its abundance of energy-dense food and sedentary lifestyles."
Joe Korner, director of communications at The Stroke Association, said the results were "shocking".
He added: "Obese people are more likely to suffer a stroke which can leave people with paralysis and communication problems.
"But you don't have to become a statistic; you can do something about it.
"By eating more healthy foods and taking regular exercise you can help reduce your risk of stroke."
The report comes as the Government launches a new campaign to urge adults to lose weight and get healthy.
The Swap it Don't Stop it part of the Change4life campaign says people do not have to give up all the things they love.
It includes suggested 'swaps' like swapping watching a favourite sport on television for taking part, increasing fibre intake by choosing brown rice over white, or swapping bigger plates for smaller ones to choose smaller portions of food.
Television and poster adverts will air from this Saturday.
Figures from the Department of Health suggest a million mothers have attempted to change their children's behaviour as a result of Change4Life.
Health Secretary, Andy Burnham, said: "In response to an urgent need to tackle the alarming rise in obesity, we launched Change4Life - not just a campaign, but a movement with a mission to encourage people to eat well, move more and live longer.
"We have surpassed all our targets for the first year and we are beginning to see the positive impact on families as they start to adopt healthier lifestyles.
"The beauty of the campaign is that it motivates everyone to get involved right down to local communities.
"For every pound we spend on the campaign, other organisations have committed to spend £3 to build momentum. This makes Change4Life a highly powerful movement."
Tim Marsh, one of the authors of today's report, said the new figures showed how obesity in adults was continuing along the same trajectory as predicted in the original Foresight report.
Peter Hollins, chief executive of the British Heart Foundation (BHF), said: "These figures predict an alarming rise in adult obesity and the knock on effect this could have on the number of people suffering from heart disease will be devastating.
"We all need to think long and hard about the long-term consequences of choices we are making today if we want to achieve a healthy old age."
Cathy Moulton, care advisor at Diabetes UK, said: "This new report paints a gloomy picture of the future state of the UK's health.
"Type 2 diabetes accounts for 90% of all cases of diabetes and obesity is one of its leading causes.
"If left undiagnosed or not controlled well, the condition can lead to heart disease, stroke, kidney failure, blindness and amputation."