WASHINGTON (Reuters) - Americans are being "bombarded" with chemicals, gases and radiation that can cause cancer, and the federal government must do far more to protect them, presidential cancer advisers said on Thursday.

Although as many as two-thirds of cancer cases are caused by lifestyle choices like smoking, poor diet and lack of exercise, the panel said many avoidable cancers were also caused by pollution, radon from the soil and medical imaging scans.

Since so little is known about the possible risks of cell phones, people would be prudent to wear headsets and make calls quickly, the two members of the panel advised.

"The panel was particularly concerned to find that the true burden of environmentally induced cancer has been grossly underestimated," they wrote in the report, available at http://www.pcp.cancer.gov.

The report is sure to stun industry and many cancer specialists. It has already delighted environmental groups, which have had hints for more than a week of the report's content.

The American Cancer Society said the report downplayed known risks that cause most cases of cancer including tobacco, obesity, alcohol, infections, hormones and sunlight.

"The report is most provocative when it restates hypotheses as if they were established facts," the group's Dr. Michael Thun said in a statement.

"For example, its conclusion that 'the true burden of environmentally (pollution) induced cancer has been grossly underestimated' does not represent scientific consensus. Rather, it reflects one side of a scientific debate that has continued for almost 30 years."

The two sitting members of the panel — the third seat is empty — are Dr. LaSalle Leffall, professor of surgery at Howard University College of Medicine in Washington and Margaret Kripke, an emeritus professor at the University of Texas M.D. Anderson Cancer Center.

It is the first time the panel has taken such a sharp turn into what had long been disputed territory — whether pollution, cell phones and even household objects, such as water bottles, can cause cancer.

Cancer is the number two killer of Americans, after heart disease. "The incidence of some cancers, including some most common among children, is increasing for unexplained reasons," the report reads.

"With nearly 80,000 chemicals on the market in the United States, many of which are used by millions of Americans in their daily lives and are un- or understudied and largely unregulated, exposure to potential environmental carcinogens is widespread," it adds.

"The American people — even before they are born — are bombarded continually with myriad combinations of these dangerous exposures," Kripke and Leffall wrote in a letter to President Barack Obama at top of the report.

"The panel urges you most strongly to use the power of your office to remove the carcinogens and other toxins from our food, water, and air that needlessly increase healthcare costs, cripple our nation's productivity, and devastate American lives."

Some specific advice from the report:

Increase, broaden and improve research regarding environmental contaminants and human health.

Raise consumer awareness of environmental cancer risks and improve understanding and reporting of known exposures.

Do more to eliminate unnecessary radiation-emitting medical tests, and to ensure that radiation doses are as low as possible without sacrificing quality.

Aggressively address the toxic environmental exposures the US military has caused, and do more to help affected military personnel and civilians.