Britain's Amir Khan says Floyd Mayweather is afraid to fight him.
Khan, 28, beat Chris Algieri in June and felt he was close to facing the unbeaten Mayweather.
But multiple world champion Mayweather, 38, will instead now fight Andre Berto on 12 September in what could be his final fight.
"I'd fight Mayweather any time. But Mayweather is scared," said Khan, "One thing about Amir Khan is, he's never afraid of anybody."
Mayweather, who beat Manny Pacquiao on points in May in Las Vegas, has opted for an opponent who has lost three of his last six fights - to Victor Ortiz, Robert Guerrero and Jesus Soto.
Bolton's Khan has won 31 of his 34 fights, and was last beaten by Danny Garcia in July last year.
"I'm disappointed not to have got the fight for what is the third time after being a frontrunner," said Khan.
"I'm going to continue working hard to establish myself at the top of the welterweight division. Floyd Mayweather has fought the biggest names throughout his career and has earned the right to face whoever he chooses.
"I wish my stable-mate Andre Berto and trainer Virgil Hunter the very best of luck in the fight. I have seen how hard Andre trains and works and know that he is ready to give Floyd a very tough night."