More than seven million new cases of diabetes, cancer, stroke and heart disease will occur in the next 20 years due to people being too fat, a coalition has warned.
The Obesity Health Alliance, whose members include the Academy of Medical Royal Colleges, the Jamie Oliver Food Foundation and Cancer Research UK, said the "worrying trend" of rising obesity is putting the nation's health at risk.
Its report suggests more than 7.6 million new cases of disease linked to being overweight or obese will be diagnosed in the UK during the next 20 years.
This includes 4.62 million cases of Type 2 diabetes, 1.63 million cases of coronary heart disease, 680,000 cases of stroke and 670,000 new cases of cancer.
Current predictions are that 76% of men and 69% of women will be overweight or obese by 2035 - around 40 million adults in the UK.
At the moment, three out of 10 adults are obese, but this is set to rise to around four in 10 by 2035. In that year, around 45% of adults in the lowest income bracket will be obese.
The alliance is calling on the Government to publish its childhood obesity strategy as soon as possible. It has been delayed since last year.
The report says small changes could make a big difference to the nation's health. Just a 1% shift in the number of people putting on extra weight each year until 2035 could avoid around 77,000 cases of disease, including 45,000 cases of Type 2 diabetes in the year 2035 alone.
It urges the Government to restrict junk food advertising on TV before the 9pm watershed, tighten online marketing rules and set targets for industry to lower the amount of sugar and fat in food.
Alison Cox, Cancer Research UK's director of prevention and member the Obesity Health Alliance, said: "These numbers are shocking. And it's difficult to think of the impact this will have on public health and an already strained NHS. Without bold action, the next generation will face more disease and live shorter lives.
"Kids are bombarded with advertisements for unhealthy food and if we are to give our children the chance for better and healthier lives, it's vital the Government's childhood obesity strategy restricts this kind of marketing."
Modi Mwatsama, Director of Policy and Global Health at the UK Health Forum and member of the Obesity Health Alliance, said: "This study is a wakeup call for the Government and shows a daunting future if no strong action is taken against the obesity epidemic.
"We can't expect industry to make changes on their own and people need help making healthier choices. Companies will have to be held accountable by Government. The Government must lead the way by creating a level playing field with independent, regulated targets for reducing the amount of sugar, fat and salt in food. Without Government action, our children face a life of disease and early death."
More than 30 organisations are members of the Obesity Health Alliance.
A Department of Health spokesman said: "This is exactly why we are going to set out our comprehensive childhood obesity strategy shortly, building on measures we are already taking like the soft drinks industry levy. The strategy will look at everything that contributes to a child becoming overweight and set out what more can be done by all."