The Richland County, South Carolina, sheriff’s department announced the decision at a news conference.

 

Sheriff Leon Lott said there was not enough evidence to charge the swimmer, who won a record eight Olympic gold medals in Beijing last year.

 

He said: “We had a photo and him saying he was sorry for inappropriate behavior.

 

"He never said, 'I smoked marijuana.’

 

"We didn’t have physical evidence.”

 

Lott said he hoped children would learn that no matter who you are, you cannot break the law.

 

He said: “Michael Phelps is truly an American hero.

 

"But even with his star status, he is obligated to obey the laws of our state.”

 

Peter Carlisle, Phelps’s agent, didn’t immediately return a phone call seeking comment.

 

Phelps, in a photograph published by London’s News of the World on February 1, was shown using a water-cooled pipe known as a bong, commonly used to smoke marijuana.

 

He has said the picture is authentic.

 

The swimmer, who used the bong three months ago, apologised for his conduct the day after the photo appeared.

 

USA Swimming banned Phelps, 23, from competition for three months on February 6, though he did nott violate any anti-doping rule.

 

Kellogg's, the world’s biggest cereal maker, said it would seek to renew a sponsorship agreement with Phelps that concludes at the end of this month.

 

Phelps, who has never failed a drug test, also ran into trouble over substance abuse after the 2004 Olympics.

 

After winning six gold medals at Athens, he was charged with under-age drinking following a traffic stop in Maryland and was sentenced to 18 months of probation.