Cell phone radiation may be carcinogenic to humans, an international panel of health experts said Tuesday, giving new weight to a growing concern that wireless devices increase a user’s chances of getting cancer.

The World Health Organization’s findings also counter a years-long lobbying campaign by the wireless phone and service industries to quash concerns about cancer risks associated with cell phone use.

The WHO’s International Agency for Research on Cancer said numerous studies showed a link between heavy cell phone users and glioma, a malignant type of brain cancer and that the agency would assign cell phone radiofrequency electromagnetic fields as category 2B:possibly carcinogenic to humans.

“The evidence, while still accumulating, is strong enough to support a conclusion and the 2B classification,” said Jonathan Samet, of the University of Southern California, who chairs the IARC working group.

“The conclusion means that there could be some risk, and therefore we need to keep a close watch for a link between cell phones and cancer risk,” Samet said.

Last February, the National Institutes of Health found that one hour of cell phone use caused increased brain activity in the area closest to a cell phone antenna.

Some five billion cell phones are used in the world, with 302 million in the United States alone. Wireless carriers generate $160 billion in revenues each year. Indeed, every metric shows Americans are talking longer, texting more and holding their wireless devices on their bodies throughout the day.

That has some public health experts and watchdogs, such as the Environmental Working Group, pushing for federal regulators and state lawmakers to create stronger safeguards for consumers.

The wireless trade group CTIA and companies such as Apple, Motorola, AT&T, Verizon and Spring Nextel have fought legislation in Maine and California that would warn consumers about the radiofrequency levels emitted by cell phones. The CTIA sued the city of San Francisco last year, which was on track to become the first city to force retailers to show the specific absorption rates of cell phones for consumers. The city’s board of supervisors shelved the initiative in May.