Phelps has won 16 Olympic medals overall, 14 of them gold
US swimming star Michael Phelps has admitted to "regrettable behaviour", after a UK newspaper published a photo of him apparently smoking cannabis.
Phelps, 23, who won eight gold medals at the Beijing Olympics last year, was pictured by the News of the World inhaling from a glass pipe.
The picture was taken in November at a party in the University of South Carolina, the newspaper said.
Phelps apologised to fans and said the incident would not be repeated.
In a statement sent to the Associated Press news agency, the swimmer acknowledged the photo was authentic.
"I engaged in behaviour which was regrettable and demonstrated bad judgment," he said.
"I'm 23 years old and despite the successes I've had in the pool, I acted in a youthful and inappropriate way, not in a manner people have come to expect from me.
"For this, I am sorry. I promise my fans and the public it will not happen again."
The US Olympic Committee said it was "disappointed" with Phelps' behaviour.
"Michael is a role model, and he is well aware of the responsibilities and accountability that come with setting a positive example for others, particularly young people," it said in a statement.
"In this instance, regrettably, he failed to fulfil those responsibilities."
USA Swimming said it hoped that "Michael can learn from this incident and move forward in a positive way".
Meanwhile, the head of America's anti-doping agency said Phelps' participation in a pilot test programme designed to increase the accuracy of doping tests could be at risk.
"For one of the Olympics' biggest heroes it's disappointing, and we'll evaluate whether he remains in that programme," Travis Tygart said.
Michael Phelps broke seven world records in Beijing, eight American records and eight Olympic records to become the most decorated male Olympian of all time with a total of 16 medals, including 14 golds.
In January he was named the United States Olympic Committee's sportsman of the year for 2008.