For more than a decade, the nonprofit Forest Stewardship Council generally has been viewed as the premier judge of whether a wood or paper product should be labeled as environmentally friendly.

But to the dismay of major environmental groups, that label, known as F.S.C., is facing a stiff challenge from a rival certification system supported by the paper and timber industry. At stake is the trust of consumers in the ever-expanding market for “green” products.

This week lawyers for ForestEthics, a nonprofit group dedicated to protecting forests, filed administrative complaints with the Federal Trade Commission and the Internal Revenue Service challenging the credibility of the rival label, known as the Sustainable Forestry Initiative, or S.F.I.