Unbeaten welterweight Floyd Mayweather Jr. has announced his retirement again, with boxing's unofficial pound-for-pound king saying he no longer has the passion necessary to fight.

Mayweather, the WBC welterweight champion, made the announcement in a letter to select media members today. The 31-year-old former U.S. Olympian (39-0, 25 KOs) hasn't fought since beating Ricky Hatton in December, but was widely expected to fight Oscar De La Hoya in September in a rematch of the richest fight in boxing history.

"This decision was not an easy one for me to make, as boxing is all I have done since I was a child," said Mayweather, the son and nephew of three of the sport's top trainers. "However, these past few years have been extremely difficult for me to find the desire and joy to continue in the sport."