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 FSC, 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 SFI.

The complaints, which challenge SFI's nonprofit status, accuse the certification program of lax standards and deceptive marketing intended to obscure the standards and the SFI's financial ties to the forest industry.

"They've essentially created a green certification system to promote their sales," said Peter Goldman, director of the Washington Forest Law Center in Seattle, the legal firm that filed the complaints on Thursday. "We believe SFI has confused the marketplace."

Karen Brandt, a spokeswoman for SFI, said that the certification program was sound and that it had met all legal requirements as a nonprofit.

SFI provides consumers with details about its labeling standards at its Web site, she added. "We're happy to provide them with the facts that support the credibility of our program," she said.

The certification program was founded in 1995 by timber and paper companies as an alternative to FSC, which was formed in 1993 by international environmental groups. (FSC includes forest industry representatives on its board.)

The FSC is bracing for a big blow if the U.S. Green Building Council, which rates buildings as environmentally sustainable under its so-called LEED system, starts accepting other types of certified wood next year, as it has proposed to do pending a vote by its membership.

Scot Horst, a senior vice president for the council, said the proposal was prompted partly by complaints that the supply of FSC-certified wood was too limited. As a compromise, LEED officials plan to formulate with their own benchmarks for wood grown and harvested in a responsible manner and to accept any certification program that meets those requirements.

LEED officials say that the SFI program certifies more forest acreage than FSC and that it showed independence by breaking away from the American Forest and Paper Association in 2007.

Still, "SFI is the 800-pound gorilla," said Rick Fedrizzi, president and chief executive of Green Building Council. "That's the group that we need to try to convince to do better in forest management."

For consumers, the array of eco-labels can be baffling. In the case of wood, Goldman noted that SFI officials proudly state at their Web site that President Barack Obama's daughters' new swing set at the White House was made from wood cut at SFI-certified forests. Yet the president's inaugural invitation was printed on FSC-certified paper.

"If the White House is confused," Goldman said, "everyone must be confused."

Urvashi Rangan, the director of technical policy at Consumers Union, which publishes Consumer Reports, said that it judged FSC to have "more rigorous" standards. She and other experts say SFI gives forest managers more latitude than FSC, which generally has more specific requirements for certification.

Yet Consumer Reports gives FSC and SFI the same rating at its GreenerChoices.com Web site: "somewhat" meaningful, as opposed to "highly" meaningful or "not" meaningful at all.

In essence, Rangan said, both programs have industry ties and could apply stricter standards in areas like preventing cutting of old-growth forests.

Certified forest products account for a small fraction of wood and paper sales in the United States, but both certification programs have reported significant growth in recent years. Today FSC certifies about 100 million acres of forest in the United States and Canada as operating in an environmentally responsible manner, compared with 175 million acres for SFI.

Some experts said the Green Building Council's move could sharply reduce demand for the FSC label if it devised watered-down requirements for wood.

"There'll be a lot less reason to get FSC," said Ben Cashore, director of the Forest Policy program at Yale University's School of Forestry and Environmental Studies. "Why would you go for the tougher program?"