Health insurance giant Anthem Blue Cross is spurning California's new insurance market for small businesses, a potential setback in the state's rollout of the federal healthcare law.

Anthem, a unit of WellPoint Inc., is California's largest insurer for small employers. The company's surprising move raised concerns about the state's ability to offer competitive rates and attract businesses to its new Covered California exchange that opens Jan. 1.

The federal Affordable Care Act left it up to health insurers to decide whether they wanted to sell in these government-run marketplaces.