The five year legal battle continues as State regulators compel food companies to paste labels on tuna cans that warn of potentially unhealthy levels of mercury.

Already, pregnant women or those who may become pregnant, nursing mothers and young children have been issued warning by The Food and Drug Administration and the Environmental Protection Agency to avoid eating the fish due to high levels of mercury, which can cause brain damage in babies.

However, tuna companies are not required to place consumer warning labels on their products by the federal government, which is why the California Attorney General's Office was back in court arguing state law requires warning labels on products that contain chemicals known to cause cancer and birth defects, which means tuna should also be labelled. The companies, on their part argue that the federal advisory is fair warning on its own.

The lawsuit filed by California pushing for labels was thrown out in 2006 by a trial judge siding with the tuna companies and scientists that 95% of the mercury found in tuna comes from natural sources. Products containing natural-occurring chemicals are not included in California's labelling law.

Justice Ignacio Ruvolo at the outset of Tuesday's hour-long hearing at a state appellate court, told the government's lawyer: 'The key finding is that wherever it's coming from, it's naturally occurring and not man-made'.

While, it is not known which way the other two judges on the panel are leaning, a decision is due within 90-days, and will no doubt be appealed to the California Supreme Court.

Despite the advisory of federal agencies advising pregnant women on avoiding tuna, nevertheless Americans are encouraged to consume it and other seafood, as part of a healthy diet.