MIAMI: Olympic champion swimmer Michael Phelps will not face prosecution over a photograph published in a British newspaper apparently showing him smoking marijuana at a party last year, a US sheriff said on Monday.

Phelps, who amazed the sports world by winning a record eight gold medals at the Beijing Games last summer, has apologized for the photograph, which showed him smoking a "bong" pipe at a student party in November at the University of South Carolina.

Reacting to the announcement by Richland County Sheriff Leon Lott that there was not enough evidence to prosecute those at the party, Phelps said he had learned "important lessons."