Tougher action – including taxing junk food – is needed by all governments if the obesity crisis is going to be tackled, experts say.
The international group of researchers, who have published a series of articles in The Lancet, said no country had yet got to grips with the problem.
They said changes in society meant it was getting harder for people to live healthy lives.
And they warned without state action, health systems could become swamped.
Obesity-related problems, such as diabetes, were now accounting for between 2% and 6% of health care costs in most countries.
But as one of the articles showed, this is likely to get worse if current trends continue.
Researchers made projections for the US and the UK – two of the developed countries with the worst rates of obesity.
They predicted obesity rates would rise from a quarter in the UK to about 40% by 2030.
Such a scenario would cost the NHS an extra £2bn a year – the equivalent of 2% of health spending.
The rise in costs would be even greater in the US, where obesity rates would rise from one in three to about one in two.
The researchers accepted that the whole of society – from the individual to industry – had a role to play in tackling the problem.
But they said governments needed to take a lead by using legislation and direct intervention to create a better environment.
They said many measures – including taxes on unhealthy food, restrictions on junk food advertising, traffic light labelling and school-based education programmes – would save money as well as benefit health.