WASHINGTON (Reuters) - Federal antitrust regulators are prepared to approve U.S. refiner Tesoro Corp.'s (TSO.N) proposed deal to buy a BP refinery in southern California, three sources told Reuters. The Federal Trade Commission, which assessed the deal to ensure that it complied with antitrust law, may announce the deal's approval as early as Friday, according to one source with knowledge of the deal. Tesoro announced in August that it had agreed to buy BP's (BP.L) Carson facility for $2.5 billion. Tesoro is the second-largest refiner in California, the nation's largest gasoline market, after Chevron Corp (CVX.N). Reporting by Diane Bartz; editing by Ros Krasny