WASHINGTON (AFP) - A batter-laden fried fish dish packs two weeks worth of harmful trans fat in a single serving and was named worst restaurant meal in America on Tuesday by a US consumer advocacy group.

The Big Catch meal, sold at the fast food chain Long John Silver's, contains 33g of trans fat and 3,700mg of sodium, said the Center for Science in the Public Interest.

People should limit themselves to 2g of trans fat daily, according to the American Heart Association, and most people should eat 1,500mg of sodium per day, according to the Institute of Medicine.

"Long John Silver's Big Catch meal deserves to be buried 20,000 leagues under the sea," CSPI executive director Michael Jacobson said in a statement announcing the group's pick of worst restaurant meal in America.

"This company is taking perfectly healthy fish - and entombing it in a thick crust of batter and partially hydrogenated oil. The result? A heart attack on a hook."

The fish is battered and fried in partially hydrogenated soybean oil, and sold with onion rings and hush puppies. Its total calorie count is rather low for a fast food meal - just 1,320, CSPI said.

But its artery-clogging trans fat is twice the level of the worst KFC dish, which had 15g of trans fat before a 2006 CSPI lawsuit led the chicken chain to stop using partially hydrogenated oil.

"Trans fat from partially hydrogenated oil is a uniquely damaging substance that raises your bad cholesterol, lowers your good cholesterol, and harms the cells that line your blood vessels," said Walter Willett, nutrition department chair at the Harvard School of Public Health.

"It might have been defensible to use hydrogenated oil in the 1980s, before trans fat's harmfulness was discovered, but no longer."

Long John Silver's introduced the "Big Catch" in May, describing it as "the largest fish we have ever offered weighing in at 7-8 ounces of 100 per cent premium Haddock caught in the icy waters of the North Atlantic." But that claim did not stand up to the scrutiny of CSPI inspectors, who picked apart the breading from the fish and said they found "an average of about four and a half ounces of actual fish and almost three ounces of oil-soaked batter."

Long John Silver's, which calls itself the "largest quick service seafood restaurant in the world," did not immediately respond to a request for comment.

CSPI said it plans to sue the chain if it continues to use partially hydrogenated oil in its deep-fryers and if it continues to misrepresent the amount of fish in the meal and the nutrition information for the side items.

The group's researchers found that the meal's onion rings were advertised to contain 7g of trans fat but actually contained 19.5g.