Whole Foods Market will require all products sold in its U.S. and Canadian stores to carry a label by 2018 saying whether they contain genetically modified organisms (GMOs), the organic and natural grocery seller said on Friday.

The U.S. is the world's largest market for foods made with genetically altered ingredients. Many popular processed foods -- including soy milk, soup and breakfast cereal -- are made with soybeans, corn and other biotech crops whose genetic traits have been manipulated, often to make them resistant to insects and pesticides.

Whole Foods -- whose 340-plus stores in the U.S., Canada and the U.K. include four in the greater Toronto area and four in Vancouver -- said the prevalence of GMOs in the U.S., coupled with a lack of labeling requirements, has made it very difficult for retailers to source non-GMO options and for consumers to identify them.

"We are stepping up our support of certified organic agriculture, where GMOs are not allowed, and we are working together with our supplier partners to grow our non-GMO supply chain," Walter Robb, co-chief executive of Whole Foods, said in a statement.

The U.S. does not require safety testing for genetically modified ingredients before they go to market. The food industry says the products are safe, but critics say there is not enough independent research to make that determination.

While the U.S. and Canada still have no GMO labelling laws, more than 60 countries do, the company said, noting several U.S. states are considering mandatory labelling initiatives.

"We're responding to our customers, who have consistently asked us for GMO labeling and we are doing so by focusing on where we have control: in our own stores," Robb said.

The announcement from Whole Foods comes as the U.S. Food and Drug Administration appears to be on the path to approving genetically engineered salmon.

At the same time, consumer groups are working at the state and federal level to require labels on products that contain GMOs.

Dozens of countries already have genetically modified food labeling requirements, with the European Union imposing mandatory labeling in 1997. Since then, genetically modified products and crops have virtually disappeared from those markets.

Whole Foods in 2009 began putting its 365 Everyday Value product line through non-GMO verification. The chain currently sells 3,300 non-GMO Project verified products, such as its organic tofu, and plans to increase that number.

The company's holdings include seven outlets in the U.K., where labeling is already required for all foods or feeds that intentionally contain or are produced from GMOs.

-- Lisa Baertlein reports on the U.S. grocery and restaurant industries for Reuters from Los Angeles.