Health experts are warning that obesity is on the rise worldwide, especially in the United States and Britain.

A new study published Friday in the medical journal, The Lancet, says 1.5 billion adults are overweight and another half-a-billion are obese. Obesity is also on the rise among the world's children, with 170 million classified as overweight or obese.

Researchers say if the current trends continue, about half of all American men and women will be obese by 2030.

The experts also predict that in Britain, obesity rates will increase during the same period from the current 26 percent for both men and women, to up to 48 percent for men and 43 percent for women.

They say the rise in obesity is likely to lead to an increase in the number of people afflicted by diabetes, heart disease, cancer and other illnesses, adding to the cost of healthcare.

The study says that in low-income countries, obesity tends to affect middle-aged adults, especially women from wealthy, urban environments. In high-income countries, it affects both sexes and all ages, but is disproportionately greater in disadvantaged groups.

Scientists say the problem is fueled by changes in the global food system which is producing more processed and affordable food than ever before and by the market economy, which stimulates overconsumption. A rise in food consumption often comes coupled with increasingly sedentary lifestyles.

Health experts are urging governments to lead the fight in reversing the obesity epidemic with policy interventions in affected communities, such as taxing unhealthy food and drinks.