ANTIOCH -- Wal-Mart's bid to expand its store here into a Supercenter is headed to court after an environmental group sued to block the plan last week.

Antioch's approval of a 33,575-square-foot expansion of the Lone Tree Way store in September violates its own municipal code and state environmental law, representatives from a coalition of environmental and labor groups said in a suit filed in Contra Costa County Superior Court.

The suit, filed by California Healthy Communities Network, will delay the company's plan to bring its first Supercenter store featuring a full-service grocery to the East Bay. Wal-Mart Stores Inc. is responsible for all city legal costs in the suit.

City Attorney Lynn Tracy Nerland could not be reached for comment Monday.

Council members said they were not surprised by the suit.

Angie Stoner, a Wal-Mart spokeswoman, said the company is frustrated.

"At a time when the city is facing potential bankruptcy and record unemployment rates, it is troubling to see, yet again, out-of-town special interest groups abusing the (environmental) process," she said.

The area surrounding the store has undergone substantial change since the environmental documents for Williamson Ranch Plaza were approved in 1998, according to the lawsuit.

As a result, Antioch should have prepared a subsequent environmental study that addresses new issues and changes to the severity of other issues, mainly the proposed expansion's effect on local grocers in the trade area, said Phil Tucker, the group's project director.

Antioch's general plan also requires leaders to consider imposing reasonable conditions on approvals to protect public health and safety.

City leaders ignored substantial evidence from three months of public meetings showing that the approval of the expansion would harm public health and welfare, according to the suit.

The City Council's approval Sept. 28 was a reversal of its initial decision to deny the project on the grounds that an environmental study for the project underestimated the potential effects on the area's economy.

The council based the approval in part on a state appellate court decision this year involving a San Diego redevelopment project that raised questions about a city's ability to consider possible environmental effects when looking at a design review application.

Attorneys for Wal-Mart and an attorney retained by the city argued that an environmental study was not required to approve the expansion.

The council did what was legally required, Councilwoman Martha Parsons said Monday.

Tucker disagrees.

"The process to approve this project was legally defective," he said. "We believe the original decision of the council, before city staff intervened, was appropriate and correct."

Wal-Mart has been trying for six years to expand its Antioch store to include a bakery, produce section and full-service deli.

The City Council narrowly defeated a larger expansion plan in 2007.

No scheduled court date for the lawsuit has been set.

Contact Paul Burgarino at 925-779-7164. Follow him at Twitter.com/paulburgarino.