The number of obese people in the world may be as high as 1.7bn, experts have calculated.
The International Obesity Taskforce (IOTF) says that the current method of calculating obesity - which gives a figure of around half of this - is flawed.
This because it fails to take into consideration the fact that Asian people seem to be more vulnerable than other racial groups to the effects of excess weight.
Professor Philip James, IOTF chairman, said the current the under-estimate had contributed to a failure of governments around the world to tackle the growing problem of obesity.
Speaking at a meeting of international obesity specialists in Monte Carlo, Professor James said that appropriate medical treatment was rarely provided to manage
Yet it was clearly established that even a modest weight reduction and improved weight control could bring health benefits, and relieve the burden on hard-pressed health systems.
Whether or not a person is overweight is currently defined by a measure called Body Mass Index (BMI), a calculation that compares height and weight.
A person with a BMI of 25 and above is defined as overweight, and somebody with a BMI of above 30 is obese.
However, an expert group of the World Health Organization, of which Professor James is a member, says that Asian people with a BMI of only 23.3 may be at increased risk of obesity-related diseases.
The IOFT estimates that a significant proportion of the 3.6 billion people who live in Asia have a BMI above this figure.
Asian people appear to be particularly at risk because they have a tendency to collect fat in the stomach area, where it is thought to have the most significant impact on health.
Professor James said: "The problem of abdominal obesity seems to be particularly marked in Asia.
"In other words you can be relatively, modestly plump but you selectively accumulate the fat in the abdomen. When it is there it amplifies the risk."
Professor James said there was a wide spectrum of risk factors related to obesity, which when viewed as a whole, have a tremendous impact on health.
"By tackling overweight through improvements in diet,
activity levels and treatment, we can have a far reaching effect on what is already a huge health burden from cardiovascular diseases and type 2 diabetes.
"It is clear that extreme forms of obesity are rising even faster than the overall epidemic and we are witnessing a real health tragedy unfolding."
In the USA the percentage of black women with morbid obesity - a BMI of 40 or more - has doubled in less than a decade to 15%.
Overall 6.3% of US women - that is one in 16 - are morbidly obese.
Professor Arne Astrup, president-elect of International Association for the Study of Obesity, said: "There is a global obesity epidemic which underpins the increasing levels of non-communicable diseases which are forecast to
explode in the next 20 years.
"It is vital that we take a more serious approach to the
treatment of the huge numbers who are obese, as well as introducing effective measures to prevent the problem getting worse."
The World Health Report 2002 estimated that more than 2.5 million deaths annually are weight related and forecast this could rise to 5 million by 2020.
Deaths directly related to obesity have been estimated at 320,000 a year in Europe and more than 300,000 in the