(Reuters) - The San Francisco Giants' postseason ambitions suffered a jolt on Wednesday when All-Star outfielder Melky Cabrera was suspended for 50 games for failing a doping test.

The 28-year-old from the Dominican Republic, the most valuable player in this year's All-Star game, had tested positive for testosterone and admitted to using a substance banned by Major League Baseball's drug prevention program.

"My positive test was the result of my use of a substance I should not have used," Cabrera, who will become a free agent after this season, said in a statement.

"I accept my suspension under the joint drug program and I will try to move on with my life.

"I am deeply sorry for my mistake and I apologize to my team mates, to the San Francisco Giants organization and to the fans for letting them down."

Cabrera's suspension was effective immediately, the baseball commissioner's office said in a statement on Wednesday.

The eight-year big-league veteran is batting .346 -- second in the National League -- with 11 home runs, 84 runs scored and 60 runs batted this season.

San Francisco (64-54) lost to the Washington Nationals 6-4 on Wednesday in their first game without Cabrera and fell a full game behind the front-running Los Angeles Dodgers in the National League West.

Los Angeles (65-53) defeated the Pittsburgh Pirates 9-3 at PNC Park in Pittsburgh.

The Giants have only 44 games left in the regular season, so Cabrera's suspension would stretch into the playoffs should they advance that far.

(Reporting by Mark Lamport-Stokes in Los Angeles; Editing by Steve Ginsburg / Ian Ransom)