(Adds comments from Campbell executive, public health lawyer)

Jan 8 (Reuters) - Campbell Soup Co is to start disclosing the presence of genetically modified organisms (GMOs) in all its U.S. products, becoming the first major food company to respond to growing calls for more transparency about ingredients in food.

The world’s largest soup maker broke ranks with peers and said late on Thursday it supported the establishment of federal legislation for a single mandatory labeling standard for GMO-derived foods and a national standard for non-GMO claims made on food packaging.

The company, which also makes Pepperidge Farm cookies and Prego pasta sauces along with its world-famous soups, said it would also withdraw from all efforts by groups opposing such measures. (bit.ly/1OeE1Md)

“This could be a watershed moment,” Michele Simon, a public health lawyer told Reuters. “There is a breaking of the ranks.”

Several activist groups have been pressuring food companies to be more transparent about the use of ingredients in their products, especially ones including GMOs, amid rising concerns about their effects on health and the environment.

Companies such as PepsiCo Inc and Kellogg Co have resisted such calls and have spent millions of dollars to defeat GMO-labeling ballot measures in Oregon, Colorado, Washington and California, saying it would add unnecessary costs.

The agrochemical and biotech industry has for years fought against efforts to pass either U.S. state or federal laws that would require food companies to adjust product labels to show GMO ingredients.

The six biggest agrochemical and biotech seed companies - Monsanto, DuPont, Dow AgroSciences, Bayer CropScience, BASF Plant Science and Syngenta - spent more than $21.5 million to help defeat a 2012 California labeling proposition, according to state election data.

In 2014, however, Vermont became the first U.S. state to pass a law requiring food companies to label GMOs on their products, which will come into effect in July.

COSTS MANAGEABLE

About three quarters of Campbell’s portfolio consisted of ingredients derived from GMOs, said Mark Alexander, senior vice president of Campbell’s Americas Simple Meals and Beverages unit.

Soybeans, sugar beets, canola and corn, the largest genetically engineered crops, are key ingredients in many Campbell soups and sauces.

Asked about the extra costs involved, the company said it would be manageable, as long as federal regulators end up introducing national labeling standards rather than leaving individual states to set different rules.

That could lead to “astronomical” costs, Alexander said, as it would involve segregation of inventories and increase supply chain costs.

Campbell’s announcement came six months after it said it would stop adding monosodium glutamate (MSG) to condensed soups for children and use non-genetically modified ingredients sourced from organic U.S. farms in its Campbell’s organic soup line for kids.

Pro-labeling campaigners such as Environmental Working Group (EWG) and Just Label It cheered Campbell’s move.

“We applaud Campbell’s for supporting national, mandatory GMO labeling,” Scott Faber, senior vice president of government affairs at EWG said.

Just Label It said Campbell’s move was a step closer to reaching the goal of a federally crafted national GMO labeling solution.

Campbell said late on Thursday that if a federal solution is not achieved, it was prepared to label all its U.S. products for the presence of ingredients that were derived from GMOs and would seek guidance from the federal food and agriculture regulators.

The Grocery Manufacturers Association (GMA), which represents more than 300 food companies opposed to mandatory GMO labeling, said it respected the rights of individual members to communicate with their customers as they choose.

However, the GMA said it was “imperative” that Congress acted immediately to prevent the expansion of a costly patchwork of state labeling laws that would ultimately hurt consumers who can least afford higher food prices. (Additional reporting by Ramkumar Iyer and Sneha Teresa Johny in Bengaluru; Editing by Don Sebastian, Sriraj Kalluvila and Bill Rigby)