Michael Phelps of the U.S. celebrates with his gold medal at the men's 100m butterfly victory ceremony at the London 2012 Olympic Games at the Aquatics Centre August 3, 2012. [Photo/Agencies]

Fresh from what he insists was his last Olympics, Michael Phelps may risk losing his new medal haul because of an ad campaign for Louis Vuitton. 

The retired top multi-Olympic champion was accused of violating the International Olympic Committee (IOC) rules after ad pictures were found on Twitter on August 13 promoting the French luxury brand that is a non-Olympic official sponsor.

Phelps' longtime agent, Peter Carlisle, denied any suggestion that his client permitted airing the pictures. "I can't count on every hand in this office the number of unauthorized uses that happen during the games period...It happens constantly at any Olympics," Carlisle said.

One of the pictures shows Phelps in a bathtub, wearing only goggles and a skimpy pair of briefs, with a Louis Vuitton tote beside the bathtub. Another picture depicts him sitting on a sofa, wearing a suit and chatting with former Soviet gymnast Larisa Latynina, who held the record for most Olympic medals before Phelps broke her record at the London Games.

Sekou Campbell, an IPR lawyer with Fox Rothschild LLP, a US national law firm said any form of release, authorized or not, is deemed as ambush marketing and is a violation to the IOC rule. Campbell is one of the members who drafted rule 40.

The 27-year-old Phelps won four gold medals and two silvers in London, raising his Olympic gold haul to 18, two silvers and two bronzes from the latest three Olympics.

The source of the leak remains unknown, according to Carlisle.