LOS ANGELES, Jan 5 (Reuters Legal) - A leading environmental advocacy group is suing the state of California's Energy Commission over its approval of a giant solar plant, underscoring the growing challenge to the nation's renewable-energy goals from within the environmental community.

The lawsuit, filed Dec. 30 in California's Supreme Court by the Sierra Club, alleges that state regulators improperly approved the plant, known as the Calico Solar Project.

The suit charges that regulators failed to fully mitigate the project's impact on rare plant and animal species, and asks the court to void approval and permits for the plant.

"We are confident in the integrity of our decision and look forward to defending our position in court," said Sandy Louey, a spokeswoman for the California Energy Commission.

At the Sierra Club, lawyer Gloria Smith said Calico is slated for "a very unfortunate site" and says her group has "very serious concerns" about the project. She said it is the first time the group has sued over a solar plant.

Conflicts between solar proponents and foes are taking on growing importance as the industry experiences a boom, particularly for California.

The lawsuit is the latest in a string of suits targeting planned solar plants, potentially setting back the development of solar energy and derailing state and federal commitments to lessening dependence on fossil fuels.

Last week, a group called La Cuna de Aztlan, which represents Native American groups such as the Chemehuevi and the Apache, filed a challenge in federal court to the federal government's approval of six big solar plants.

In December, the Quechan Indian tribe won an injunction blocking construction of the Imperial Valley solar project, under development near California's border with Mexico by NTR's Tessera Solar.

The Calico plant was also under development by Tessera until the company sold the plant last month to K Road Sun, a subsidiary of New York investment firm K Road Power. Tessera has been struggling to find funding for its plants, which cost about $2 billion.

Other companies with plants under development that are raising environmental concerns include First Solar Inc and SunPower Corp.

Last year, just three new utility-scale solar plants serving California came online, according to the California Public Utilities Commission. Now, there are more than 40 plants with contracts or pending contracts with the state's utilities under development.

While fostering renewable energy has become an important federal and state goal, proposed plants are meeting increasing resistance from groups that believe the plants will do irreparable harm to threatened or endangered plants and animals or historic areas.

In the Calico case, the California Energy Commission has 10 days to file a response. A judge should decide about a month after the suit was filed if the Sierra Club can proceed with its action.

The case is Sierra Club v. California Energy Resources Conservation and Development Commission (Tessera Solar), California Supreme Court, No. S189387. The docket lists Sierra Club attorneys Travis Ritchie and Gloria Smith as counsel for the petitioner. Michael Levy of the California Energy Commission is listed as counsel for the respondent. Ella Foley-Gannon of Bingham McCutchen in San Francisco is listed as counsel for Tessera Solar North America Inc.

The federal case filed last week is La Cuna de Aztlan Sacred Sites Protection Circle Advisory Committee et al v. U.S. Department of the Interior et al, U. S. District Court, Southern District of California, No. 3:10-cv-02664. The complaint lists Cory Briggs and Mekaela Gladden of Briggs Law Corp in Upland, California, as counsel for La Cuna de Aztlan. Department of Justice attorney David Glazer is listed as counsel for the government defendants. George Caplan, Paul Gelb and Adam Thurston of Drinker Biddle & Reath in Los Angeles are listed for intervenor Genesis Solar. Christopher McGrath of Paul, Hastings, Janofsky & Walker is listed for intervenor-defendant Palo Verde Solar I.

The case against the Imperial Valley solar project is Quechan Tribe of the Fort Yuma Indian Reservation v. U.S. Department of the Interior et al, U.S. District Court, Southern District of California, No. 10-cv-2241.

(Reporting by Sarah McBride of Reuters; Additional reporting by Terry Baynes of Reuters Legal)