Floyd Mayweather Jr., widely regarded as the world's best fighter pound-for-pound, announced his retirement from the sport for the second time in 13 months.
In May last year, Mayweather said he would quit boxing after beating fellow American Oscar De La Hoya on a split decision in Las Vegas to claim the World Boxing Council super welterweight title.
He subsequently changed his mind and had been scheduled to meet De La Hoya in an eagerly-anticipated re-match this September after beating Britain's Ricky Hatton in a WBC welterweight title bout in December.
"It is with a heavy heart that I write you this message today," Mayweather, 31, said in a letter to the boxing media. "I have decided to permanently retire from boxing.
"This decision was not an easy one for me to make as boxing is all I have done since I was a child. However, these past few years have been extremely difficult for me to find the desire and joy to continue in the sport."
Mayweather had not fought since retaining his WBC welterweight title with a 10th round stoppage of Hatton, the American setting up the win with two knockdowns.
In the biggest match-up of undefeated welterweights since De La Hoya and Felix Trinidad clashed in 1999, Mayweather dropped Hatton to the canvas midway through the round before the referee stepped in to end the fight when the Briton went down again moments later.
The unbeaten American improved his career record to 39-0 with 25 knockouts.
"I have said numerous times and after several of my fights over the past two years that I might not fight again," Mayweather added.
"At the same time, I loved competing and winning and also wanted to continue my career for the fans, knowing they were there for me and enjoyed watching me fight.
"However, after many sleepless nights and intense soul-searching I realised I could no longer base my decision on anything but my own personal happiness, which I no longer could find.
"So I have finally made up my mind, spoken to my family, particularly my mother, and made my decision."
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