Story highlights Phelps testified before US congressional hearing

"Something has to be done now," he says

Swimmer has won 23 Olympic golds

(CNN) He is the most decorated Olympian of all time, but Michael Phelps believes he has never competed against a clean international field.

The retired American swimmer, who won 23 Olympic gold medals, urged greater international policing of doping in sport.

He told a US congressional hearing on improving anti-doping measures "the time to act is now."

"I don't believe I've stood up at an international competition and the rest of the field has been clean," the 31-year-old said Tuesday. "I don't think I've ever felt that.

"I know that when I do stand up in the US, I know we're all clean because we're going through the same thing.

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