By Will Dunham

WASHINGTON (Reuters) - Cancer is on pace to supplant heart disease as the No. 1 cause of death worldwide in 2010, with a growing burden in poor countries thanks to more cigarette smoking and other factors, global health experts said on Tuesday.

Globally, an estimated 12.4 million people will be diagnosed with some form of cancer this year and 7.6 million people will die, the U.N. World Health Organization's International Agency for Research on Cancer said in a report.

"The global cancer burden doubled in the last 30 years of the 20th century, and it is estimated that this will double again between 2000 and 2020 and nearly triple by 2030," according to the report.

By 2030, 26.4 million people a year may be diagnosed with cancer, with 17 million people dying from it, the report forecast.

In men, lung cancer was the most common form in terms of new cases and deaths, while breast cancer was the most common type among women in new cases and deaths, according to the report. More men than women get cancer and die from it.

"This is going to present amazing problems at every level in every society worldwide," the IARC's Peter Boyle said at a news conference.

In the near term, cancer is expected to bypass heart disease as the leading killer globally in 2010, American Cancer Society Chief Executive Officer John Seffrin said. Cancer currently accounts for about one in eight deaths worldwide.

Trends that will contribute to rising cancer cases and deaths include the aging of populations in many countries -- cancer is more common in the elderly -- and increasing rates of cigarette smoking in poor countries.

Some rich countries have made progress in cutting cigarette smoking, which causes most cases of lung cancer as well as many other illnesses. In the United States, the most recent figures show that for the first time since records have been kept less than 20 percent of adults were smokers in 2007.

However, cigarette companies are finding new customers in developing countries. Seffrin noted that 40 percent of the world's smokers live in just two nations -- China and India.

Decades ago, cancer was considered largely a problem of Westernized, rich, industrialized countries. But much of the global burden now rests in poor and medium-income countries.

Many of these countries have limited health budgets and high rates of communicable diseases, while cancer treatment facilities are out of reach for many people and life-saving treatments are seldom available, Boyle said.

"There are more deaths in the world from cancer than from AIDS, tuberculosis and malaria combined," Boyle said.

At the same time, progress against cancer has been reported by authorities in such places as the United States and Europe.

For example, health authorities in the United States reported last month that cancer diagnosis rates are now dropping for the first time in both men and women and previous declines in cancer death rates are accelerating.

They attributed the progress to factors such as regular screening for breast and colorectal cancer, declining smoking rates and improved treatments.

Cancer-prevention opportunities exist in countries of any income level, Boyle said, noting that many types of cancer are caused by individual behaviors such as smoking.

Some other "modifiable risk factors" for cancer that Boyle cited included alcohol consumption, too much exposure to sunlight, lack of physical activity and obesity.