SAN FRANCISCO — Superman belongs to Warner Bros., according to the latest legal victory granting the film and television studio complete commercial control of the lucrative Superman franchise.

A three-judge panel of the Ninth US Circuit of Appeals unanimously ruled Thursday that the heirs of Superman’s cocreator Jerome Siegel must abide by a 2001 letter written by the family’s attorney accepting Warner Bros.’ offer for their 50 percent share of Superman. Though the five-page letter was never formalized into a contract, the appeals court said it was still binding.