Phelps 'sorry'

"I engaged in behaviour which was regrettable and demonstrated bad judgment," Phelps, the winner of a record eight gold medals at the Beijing Olympics last year, said.

US officials said the news was disappointing but stressed that smoking marijuana out of competition was not an anti-doping matter at this point.

"Obviously it is very disappointing and a terrible decision by Michael," US Anti-Doping Agency chief executive Travis Tygart told Reuters.

"To a certain extent, he let down the world."

But Tygart added: "Marijuana, right or wrong, is not prohibited out of competition. Obviously if he showed up at an event and tested positive we would take full action against him."

The US Olympic Committee and USA Swimming also expressed disappointment.

"Our Olympic champions are role models who are looked up to by people of all ages, especially young athletes who have their own aspirations and dreams," USA Swimming stated.

"That said, we realise that none among us is perfect. We hope that Michael can learn from this incident and move forward in a positive way."

The USOC had a similar response.

"We are disappointed in the behaviour recently exhibited by Michael Phelps," USOC said in a statement.

"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. In this instance, regrettably, he failed to fulfil those responsibilities.

"We are confident that, going forward, Michael will consistently set the type of example we all expect from a great Olympic champion."

The News of the World said the photograph of Phelps was taken at a house party being held by students at the University of South Carolina in Columbia last November.

The newspaper did not say Phelps was smoking marijuana but said the glass pipe the swimmer was photographed using generally was used to smoke the weed.

Phelps, who has never failed a doping test, promised in his statement there would be no repeat of his behaviour.

"I'm 23 years old, and despite the successes I have had in the pool, I acted in a youthful and inappropriate way, not in a manner that people have come to expect from me," Phelps said.

"For this, I am sorry. I promise my fans and the public - it will not happen again."

Tygart said that while out-of-competition usage is not prohibited, "it is still a grievous behaviour that he obviously will pay a huge penalty for (image-wise).

"He'll suffer by all the young kids out there, he will have to look in the eye again and acknowledge such a terrible mistake, terrible decision."

Phelps won eight events in Beijing to take his career gold medal tally to 14, making him the most successful competitor since the modern Olympics started in 1896.