Toyota Motor Corp. is assessing the cost of shutting the California auto plant it used to share with General Motors Corp. and is leaning toward closure instead of funding the factory on its own, two sources familiar with the plan said.

The carmaker is likely to close New United Motor Manufacturing Inc. because the cost of keeping the factory open will rise, according to the sources.

Toyota was already grappling with excess North American assembly capacity when bankrupt GM junked the 25-year-old joint venture June 29. Severance packages for the factory's unionized workers would be a substantial portion of the overall closing cost, one of the sources said.

Mike Michels, a spokesman for Toyota's U.S. sales unit, said the company hasn't made a decision on the fate of the Fremont, Calif., plant known as NUMMI.