Britain may become the fat man of Europe within a decade after the world's most comprehensive obesity study found almost four in 10 people will be dangerously overweight by 2025. For the first time there are now more adults in the world classified as obese than underweight, laying bare what scientists warned was a global obesity crisis.

The study conducted by researchers from Imperial College London and published in The Lancet, compared body mass index (BMI) for almost 20 million adult men and women across the world from 1975 to 2014. It found obesity in men had tripled and more than doubled in women over the past four decades.

Globally, this translates as more than one in 10 men (266 million) and one in seven women (375 million) currently classed as obese. It also means the world's population has become heavier by around 1.5kg in each subsequent decade since 1975.

Lead author Professor Majid Ezzati described it as an "epidemic of severe obesity" and urged governments to act. He said initiatives such as the recently announced sugar tax should go further to include duty on highly processed foods and subsidies on healthier options.

Analysis also showed more obese men and women now live in China and the USA than in any other country. However, the USA still has the highest number of severely obese men and women in the world.

In the UK, obesity rates are 28.4% for women – the second highest in Europe behind Malta – and 26.2% for men, the worst in the continent. By 2025, it is predicted to become the fattest nation in Europe, with almost 40% of adults obese.

Ezzati said: "The number of people across the globe whose weight poses a serious threat to their health is greater than ever before. And this epidemic of severe obesity is too extensive to be tackled with medications such as blood pressure-lowering drugs or diabetes treatments alone, or with a few extra bike lanes.

"We need coordinated global initiatives – such as looking at the price of healthy food compared to unhealthy food, or taxing high sugar and highly processed foods – to tackle this crisis."

Researchers predicted if global trends continue, by 2025, 18% of the world's men and 21% of women will be obese. It could mean nations facing an epidemic of costly health conditions such as diabetes, heart disease and cancer.

This is especially true for English-speaking high-income countries, which suffer a worse weight problem than other nations. Almost a fifth of the world's obese adults (118 million) live in Australia, Canada, Ireland, New Zealand, UK, and the US.

Other key findings in the report included:

  • The country with the highest average BMI was American Samoa, where the average citizen is classed as obese;
  • American men and women had the highest BMI of any high-income country;
  • Japanese men and women had the lowest BMI in the high-income world;
  • Timor-Leste, Ethiopia, and Eritrea had the lowest BMI in the world;
  • Swiss women and Bosnian men had the lowest BMIs in Europe; and
  • Morbid obesity, where a person's weight interferes with basic physical functions such as breathing and walking, now affects around 1 % of men in the world, and 2% of women. In total, 55 million adults are morbidly obese.

Jamie Blackshaw, national lead for obesity and healthy weight, Public Health England, said: "People who are overweight and obese suffer life-changing consequences and it costs the NHS more than £6bn ($8.6bn) a year. The causes of obesity are complex and the environment we live in encourages poor diets and low levels of physical activity.

"There is no single solution, we have to address the many factors that drive up obesity levels. We all – government, industry, local authorities and the public – have a role to play in that.

"That's why we're supporting the government to develop its childhood obesity strategy, we're running the world's first national diabetes prevention programme and we're currently piloting, with local councils and Leeds Beckett University, a whole systems approach to tackling obesity."

The study also looked at the number of people who are underweight. Over the past four decades, rates had fallen from 14% to 9% in men, and 15% to 10% in women. But the issue remains a significant challenge for countries like India and Bangladesh, where nearly a quarter of adults are still classed as underweight.