WASHINGTON (Reuters) - Pregnant women should not eat canned tuna because it may contain harmful levels of mercury, Consumer Reports magazine said on Monday, taking a more cautious approach than that recommended by the U.S. government.

Government tests found instances where canned light tuna had as much mercury, a potentially harmful heavy metal, as white tuna, also known as albacore, according to the magazine's latest issue.

High levels of mercury in the bloodstream may harm developing nervous systems, according to the Food and Drug Administration. Fish and shellfish are the main sources of mercury exposure for humans.

Since March 2004, the Food and Drug Administration and the Environmental Protection Agency have recommended women who are pregnant, planning to become pregnant or nursing should eat no more than 6 ounces (170 grams) of albacore tuna a week.

But the government says it is safe to eat up to 12 ounces (340 grams) -- the amount of fish in two meals -- per week of fish and shellfish low in mercury, such as shrimp, salmon, light tuna, catfish and pollock.

Consumer Reports said 6 percent of canned light tuna tested by the FDA "contained at least as much of the metal -- in some cases more than twice as much -- as the average albacore." Most of the cans had only one-third as much mercury.

Jane Halloran, director of food policy initiatives for Consumers Union, said due to concerns that both types of tuna showed instances of higher levels of mercury, it decided to recommend pregnant women eat neither. Consumers Union publishes Consumer Reports.

The FDA said its extensive testing of canned light tuna showed an average mercury content of 0.12 parts per million, one-tenth the level found in species that it advises pregnant women to avoid.

The FDA said there is no research suggesting harm from an occasional serving of tuna that has a higher than average level of mercury.

The U.S. Tuna Foundation trade group said the magazine was overreacting to a minor problem. It said the nutritional benefits of seafood easily outweigh the risk posed by "trace amounts of mercury" and said scientific research shows the federal guidelines on consumption are sound.

Two consultants to the tuna industry said the Consumer Reports advice was not consistent with the conclusions of most researchers.

Dr. Joshua Cohen of the Tufts New England Medical Center said mercury risks to unborn babies "depends not on exposure on any given day but on the average exposure over several weeks."

The three major U.S. tuna companies are Del Monte Foods, maker of StarKist tuna; Bumble Bee Seafoods, a unit of Connors Brothers Income Fund of Canada; and Tri-Union Seafoods, maker of Chicken of the Sea.

Consumer Reports said the higher levels of mercury sometimes found in canned tuna might come from yellowfin tuna, which tends to carry more mercury than skipjack, which is usually used in light tuna.

Consumer Reports said its story would be available on the Internet at http://www.ConsumerReports.org.