(Reuters) - The U.S. Senate on Wednesday blocked a bill that would nullify state and local efforts to require food makers to label products made with genetically modified organisms, or GMOs, as the industry races to stop Vermont's law from taking effect on July 1.

The proposed legislation from Republican Senator Pat Roberts of Kansas comes amid growing calls for transparency in the U.S. food supply. Labeling advocates have criticized the bill as toothless because it leaves the decision to disclose GMO ingredients to the companies whose products contain them.

Senate Bill 2609 is known as the Biotech Labeling Solutions Act by supporters and the Deny Americans the Right to Know, or DARK, Act by opponents. A procedural vote on Wednesday failed to reach the necessary 60 votes to advance the bill in the Senate, with 49 yes votes and 48 no votes.

Roberts vowed to keep fighting as the July 1 deadline looms for Vermont's labeling requirement to take effect.

"I remain at the ready to work on a solution," Roberts said.

The United States is the world's largest market for foods made with genetically altered ingredients. Many popular processed foods are made with soybeans, corn and other biotech crops whose genetic traits have been manipulated, often to make them resistant to insects and pesticides.

Major food, farm and biotech seed companies spent more than $100 million in the United States last year to battle labeling efforts, according to a lobbying disclosure analysis from the Environmental Working Group, which opposes the Senate measure.

Opponents to GMO labeling efforts include trade groups such as the Grocery Manufacturers Association, whose members have included PepsiCo Inc and Kellogg Co, and BIO, which counts Monsanto Co, Dow AgroSciences, a unit of Dow Chemical Co, and other companies that sell seeds that produce GMO crops among its members.

They say labeling would impose speech restrictions on food sellers, burden consumers with higher costs and create a patchwork of state GMO labeling policies that have "no basis in health, safety or science."

But companies such as Whole Foods Market Inc, Chipotle Mexican Grill Inc and Campbell Soup Co already have begun labeling or abandoning GMOs rather than waiting for government action.

(Reporting by Lisa Baertlein in Los Angeles and Karl Plume in Chicago; Editing by Lisa Von Ahn)