A former rival of Michael Phelps has questioned the American swimming great after he claimed he never swam in a field clean of dopers.

Phelps testified yesterday before a congressional hearing on improving anti-doping measures, delivering the message that he doesn't believe the Olympics and other competitions are clean and that athletes don't believe in the testing system that's in place.

Phelps, who has won 28 Olympic medals, said athletes get "disillusioned" when they see others cheat, he and asked the House Energy and Commerce Subcommittee on Oversight and Investigations to help "ensure the system is fair and reliable."

"I don't believe that I've stood up at international competitions and the rest of the field has been clean," Phelps said during the hearing. "I don't believe that. I don't think I've ever felt that. I know that when I do stand up in the U.S., I know we're all clean because we're going through the same thing. Internationally I think there has to be something done, and it has to be done now."

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Serbian Milorad Cavic, who was narrowly pipped for gold by Phelps at the Beijing Olympics, questions the timing of Phelps speaking out against doping and only commenting on the issue now that he has retired.

In an open letter Cavic, who took silver in the 2008 100m butterfly, says he's not calling a Phelps a drug cheat but went onto the state the American's 'recovery rate was nothing short of science fiction'.

"Doping has been a problem and it's only getting worse. I, too, don't know what to tell my son, nor would I wish that my son ever be half as good as I was knowing what he'll face tomorrow," Cavic wrote in an open letter posted on his Twitter account.

"I could recall Lance Armstrong getting tested 3x in one day, and never failed once, but that's not the problem. At the moment, we're not able to detect new drugs and advanced methods of doping. Why you're seeking reform now that you're retired, and never before supported blood passports, is beyond all of us, perhaps even convenient.

I'm not suggesting you're a cheat, you've gradually improved your times throughout your career, but your recovery rate is nothing short of science fiction....We all just wish we could understand it. Anyway, I really do hop that you'll sticks with this because in case our sons go pro some day, I;d like to think you made a difference. #nevertoolate"