WASHINGTON (Reuters) - The U.S. Supreme Court upheld Monsanto Co's biotech seed patents on Monday, dealing a blow to a group of organic farmers and other activists trying to stop the biotech company from suing farmers if their fields contain a few plants containing the company's genetically modified traits.

The Organic Seed Growers and Trade Association and a group of dozens of organic and conventional family farmers, seed companies and public advocacy interests sued Monsanto in March 2011. The suit sought to prohibit the company from suing farmers whose fields became inadvertently contaminated with corn, soybeans, cotton, canola and other crops containing Monsanto's genetic modifications.

Monsanto has genetically engineered its specialty seeds to withstand dousings of glyphosate, the main ingredient in the company's Roundup herbicide.

The group asked Monsanto for a pledge not to sue, but the company refused, saying: "A blanket covenant not to sue any present or future member of petitioners' organizations would enable virtually anyone to commit intentional infringement."

The biotech crops are widely used throughout the United States. Monsanto has sued more than 100 farmers for patent infringement, winning judgments against those found to have made use of its seed without paying required royalties.

The group of Monsanto critics lost in district court and in the U.S. Court of Appeals for the Federal Circuit.

"Monsanto never has and has committed it never will sue if our patented seed or traits are found in a farmer's field as a result of inadvertent means," said Kyle McClain, the company's chief litigation counsel.

"The lower courts agreed there was no controversy between the parties," McClain added, "and the Supreme Court's decision not to review the case brings closure on this matter."

OSGATA President Jim Gerritsen said he was disappointed in the high court's refusal to hear the case.

"The Supreme Court failed to grasp the extreme predicament family farmers find themselves in," said Gerritsen, a Maine organic seed farmer. "The Court of Appeals agreed our case had merit. However, ... safeguards they ordered are insufficient to protect our farms and our families."

The case is Organic Seed Growers and Trade Association, et al., v. Monsanto Company, et al. Supreme Court Case No. 13-303.

(Reporting by Diane Bartz and Carey Gillam; Editing by Lisa Von Ahn and David Gregorio)