The World Trade Organization has ruled in favor of Mexico in a dispute over tuna with the United States that began more than 20 years ago, a high-ranking source close to the negotiations told Reuters.

The WTO's decision lifts restrictions for Mexico to ship the fish to the United States, barred because it does not carry a "dolphin safe" label.

The United States stopped selling Mexican tuna in 1991, citing complaints that the fishing techniques used by its neighbor were hurting the local dolphin population.

Mexico has said those concerns are unfounded because its fishermen follow international standards. Mexico filed a complaint with the WTO in 2009 claiming the labeling rules accounted for unfair trade practices.

Both countries were told about the WTO decision in Mexico's favor on July 8, the source said, speaking on condition of anonymity, but the final document has yet to be made public.

"By the end of 2012 or at the beginning of 2013, Mexican tuna should be entering the U.S. market," the source said. "What we have to wait for now is that the WTO translates (the document) into French and Spanish."

While the United States can appeal the WTO decision, the source said it was unlikely that the trade organization's ruling would change.

The U.S. Trade Representative's office confirmed the WTO panel issued a report on the case but said it could not comment because the results are still confidential.

"The United States will continue to vigorously pursue the objectives of the dolphin safe labeling provisions," U.S Trade Representative spokeswoman Nkenge Harmon said. (Reuters)