MANNY Pacquiao's promoters say his prospective bout with Floyd Mayweather Jr. is off after mediation failed to resolve the fighters' differences, scrapping what was likely to have been the richest fight in boxing history.

The bout was slated for March 13 at the MGM Grand Garden in Las Vegas, but Top Rank said late Wednesday it couldn't reach an agreement with Golden Boy Promotions, which represents Mayweather.

The promoters went into mediation on Tuesday to resolve their differences over drug testing for the fight. Both sides claimed every other detail had been agreed.

The fighters were close to a deal after the mediation, but subsequent discussions Wednesday led Top Rank to declare the fight canceled. (AP)