The Mercury Policy Project, based in Vermont, said it randomly bought 48 cans of different brands of albacore tuna from grocery stores across the nation and had them tested by an independent laboratory. Some of the results were also tested a second time by a laboratory used by the food industry.

More than 6 percent of the samples contained mercury at or above the U.S. Food and Drug Administration's so-called "action level" of 1 part per million (ppm). The results showed average mercury levels of more than 0.5 ppm for all the samples, said Michael Bender, director of the activist group.

"Because the FDA halted testing of canned tuna for mercury in 1998 to save money and because the industry keeps its results secret, some parents are unknowingly exposing their children to high mercury levels," Bender said in a statement.

White or albacore tuna accounts for about one-third of the canned tuna sold in the United States. The American Heart Association and other health experts have encouraged Americans to eat more tuna and other fish for the Omega-3 fatty acids and other nutritional benefits.

The tuna industry dismissed the activist group's findings, saying they were based on too small a sample to be valid.

"It's very clear that pregnant women can safely consume up to 12 ounces of a variety of fish each week without any problems whatsoever," said Randi Thomas, a spokeswoman for the U.S. Tuna Foundation.

She said the foundation's testing showed mercury well below the FDA's 1 ppm threshold for action. Thomas said the FDA does regularly test tuna for mercury and has found an average of 0.17 ppm.

At least 11 states have issued advisories warning that pregnant women and children should limit their consumption of canned tuna. Some states have also alerted consumers that white albacore tuna may contain more mercury than "light" tuna.

Earlier this year, Britain's Food Standards Agency urged pregnant women to limit their consumption of tuna to no more than two cans per week due to the mercury risk to their unborn children. California has sued several grocery chains for failing to warn consumers about the risk of mercury in tuna, swordfish and shark.