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

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

The bill is known as the Biotech Labeling Solutions Act by supporters and the Deny Americans the Right to Know, or DARK, Act, by opponents. Proponents of the legislation on Wednesday failed to obtain 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 deadline looms for Vermont's law - which requires labels on food products containing GMOs - to take effect.

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

Connecticut and Maine have passed GMO labeling laws similar to Vermont's, but those are on hold until other states enact similar measures. GMO labeling bills are pending in 31 states, according to the National Conference of State Legislatures.

The United States is the world's largest market for foods made with genetically altered ingredients. Many 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 fighting labeling efforts, according to the Environmental Working Group, which opposes the Roberts measure.

GMO labeling foes 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."

Some high-profile companies are not waiting for federal lawmakers to act, however. Whole Foods Market Inc, Chipotle Mexican Grill Inc and Campbell Soup Co already have begun labeling or abandoning GMOs. (Reporting by Lisa Baertlein in Los Angeles and Karl Plume in Chicago; Editing by Lisa Von Ahn and Steve Orlofsky)