The study states that almost four out of 10 men and women in Britain (38 per cent) will be obese by 2025.
The research suggests Britain will have the highest proportion of fat women in Europe followed by Ireland (37 per cent) and Malta (34 per cent) and the most fat men along with Ireland and then Lithuania (36 per cent).
An even higher proportion of American women (43 per cent) and men (45 per cent) are predicted to be obese in 2025.
Currently the UK has the third highest average BMI in Europe for women equal to Ireland and the Russian Federation - all around 27.0 kg/m².
It also has the tenth highest for men along with Greece, Hungary, and Lithuania - all around 27.4 kg/m².
Europe’s fattest women are in Moldova (27.3 kg/m²) with the slimmest in Switzerland (23.7 kg/m²).
Men in Cyprus, Ireland, and Malta are the fattest (27.8 kg/m²) but Bosnian and Dutch men the slimmest (both around 25.9 kg/m²).
In the past 40 years there has been an explosion in the numbers of people that are obese rising from 105 million in 1975 to 641 million in 2014.
Almost a fifth of the world’s obese adults - 118 million - live in just six high-income English-speaking countries - Australia, Canada, Ireland, New Zealand, UK, and USA.
More than a quarter - 27.1 per cent or 50 million - of the world’s severely obese people also live in these countries.
The finding comes as Imperial College London said there will be for the first time ever more obese people in the world than underweight by 2025.
The study, published in The Lancet, was the most comprehensive analysis of trends in body mass index (BMI) to date.
It found the age-corrected proportion of obese men has more than tripled from 3.2 per cent to 10.8 per cent and the proportion of obese women has more than doubled from 6.4 per cent to 14.9 per cent since 1975.
At the same time, the proportion of undernourished people fell more modestly-by around a third in both men from 13.8 per cent to 8.8 per cent and women from 14.6 per cent to 9.7 per cent.
Over the past four decades, the average age-corrected BMI increased from 21.7kg/m² to 24.2 kg/m² in men and from 22.1kg/m² to 24.4 kg/m² in women, equivalent to the world’s population becoming on average 1.5kg (3.3lbs) heavier each decade.
If the rate of obesity continues at this pace, by 2025 roughly a fifth of men (18%) and women (21%) worldwide will be obese, and more than 6 per cent of men and 9 per cent of women will be severely obese (35 kg/m² or greater).
Professor Majid Ezzati from the School of Public Health said: “Over the past 40 years, we have changed from a world in which underweight prevalence was more than double that of obesity, to one in which more people are obese than underweight.
“If present trends continue, not only will the world not meet the obesity target of halting the rise in the prevalence of obesity at its 2010 level by 2025, but more women will be severely obese than underweight by 2025.
“To avoid an epidemic of severe obesity, new policies that can slow down and stop the worldwide increase in body weight must be implemented quickly and rigorously evaluated, including smart food policies and improved health-care training.”
The study also found women in Singapore, Japan, and a few European countries including Czech Republic, Belgium, France, and Switzerland had virtually no increase in average BMI (less than 0.2 kg/m² per decade) over the 40 years.
Island nations in Polynesia and Micronesia have the highest average BMI in the world reaching 34.8 kg/m² for women and 32.2 kg/m² for men in American Samoa.
In Polynesia and Micronesia more than 38 per cent of men and over half of women are obese.
Timor-Leste, Ethiopia, and Eritrea have the lowest average BMI in the world.
Timor-Leste was the lowest at 20.8 kg/m² for women and Ethiopia the lowest at 20.1kg/m² for men.
More than a fifth of men in India, Bangladesh, Timor-Leste, Afghanistan, Eritrea, and Ethiopia, and a quarter or more of women in Bangladesh and India are still underweight.