VIENNA (Reuters) - Obesity is set to become the biggest disease of the century and health experts said curbing childhood obesity could prevent millions of cancer cases and other related diseases.

Tobacco is the leading preventable cause of cancer-related deaths but obesity, which is linked to kidney, colon and breast cancers, is not far behind.

"After smoking, the obesity epidemic is the single most important modification to reduce cancer," Professor Jaap Seidell, a Dutch epidemiologist told an international obesity conference.

Health experts say reducing weight through better diets and more exercise could cut the number of cancer cases by 30 to 40 percent, three to four million cases, worldwide each year.

In the United States, obesity is linked to 300,000 deaths annually - second only to 400,000 tobacco-related deaths.

"Not everyone realises fully that being overweight and obesity contribute significantly to certain cancers and are not just implicated in heart disease and diabetes," said Professor Philip James, the chairman of the International Obesity Task Force (IOTF).

The World Health Organisation (WHO) estimates that one in four kidney and gallbladder cancer cases, one in ten colon and one in 12 breast cancers in postmenopausal women, are attributable to obesity and being overweight.

Experts attending the 11th European Congress on Obesity said more than 300 million people around the world are obese. Obesity is increasing globally in developed and developing countries and at an alarming rate in children.

"People are getting heavier and heavier, younger and younger and that increases the prevalence of obesity over time," said Seidell.

If measures are not taken to ease the problem, experts predict the numbers could double in the next 20 years. The biggest rise in childhood obesity has been in the United States but European countries are following the trend. About one in seven children in France and one in five in Italy is overweight. Research has shown obesity in children is more common in large cities where youngsters exercise less because of a lack of safe places to play, and among families with low education levels. It is also more prevalent in small families and if the mother works outside the home.

Although genetics plays a part, poor diets, lack of exercise and too much time in front of the television and computer are the main factors contributing to the growing number of obese children.

New anti-obesity drugs can prevent the absorption of fat or help reduce how much people eat but Seidell said prevention is the key element in reducing obesity, particularly in children.

"The evidence linking being overweight and some forms of cancer is growing very strong. It is important that this is now brought home to people in a way that they can clearly understand," said Seidell.

"How we deal with childhood obesity is the biggest single public health challenge of this century." More than 2,500 doctors, researchers and public health officials from 24 countries are attending the four-day conference on the medical, psychological and economic impact of obesity.