SAN FRANCISCO (AP) - A state judge ruled that California cannot require the nation's three largest tuna companies to place mercury warning labels on their cans.

State Attorney General Bill Lockyer sued the tuna canners two years ago to enforce Proposition 65, a 1986 state law requiring consumer warnings about reproductive toxins such as mercury, which has been linked to developmental problems in children.

With some studies showing that tuna contains potentially unsafe levels of mercury, health advocates complain that tuna companies and government agencies haven't done enough to warn consumers about the risks.

In a ruling issued late Thursday, Superior Court Judge Robert Dondero said the state's attempt to enforce the law conflicts with federal policies on tuna consumption.

The judge also found that the mercury levels in the companies' tuna were not high enough to require labels. In addition, he ruled that virtually all the mercury in canned tuna was naturally occurring, exempting them from the warning.