Michael Phelps, once referred to by his coach Bob Bowman as “a solitary man” and who last year managed to keep news of his marriage secret until four months after the event, is not a man given to grand public pronouncements. Instead the greatest Olympian of all time prefers to live his life out of the public glare.

“Before Rio I would have my headphones on and not really talk to anybody,” he memorably commented in an interview ahead of the 2016 Olympic Games, in an admission that neatly summarised his character.

And yet Phelps changed during those Games. More opinionated and forthright, he made headlines when he spoke out against drugs cheats, stressing that the inclusion of athletes at Rio who had served bans for doping was “heart-breaking” for those who completed clean.

On Tuesday, speaking as an ambassador on behalf of Olympic sport, Phelps broke from cover once again to make his strongest statement yet on the fight against doping.

Appearing in front of a US House of Representatives Committee, the 31-year-old passionately urged lawmakers to push for sweeping anti-doping reforms in sport and told them that he had never been totally confident that he was competing only against clean athletes during his glittering career.

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His frustration at a perceived lack of progress in the battle against performance enhancing drugs in sport was palpable as he called on the World Anti-Doping Agency (Wada) to do more to tackle drugs cheats.

"I don't believe I've stood up at an international competition and the rest of the field has been clean," said Phelps, who won a staggering 28 Olympic medals, 23 of those gold. “I don't think I've ever felt that. I know when I do stand up in the U.S. I know we're all clean because we go through the same thing.

“Throughout my career I have thought that some athletes were cheating and in some cases those suspicions were confirmed. Given all the testing I and others have been through, I have a hard time understanding this.

“If we allow our confidence in fair play to erode, we will undermine the power of sport and the goals and dreams of future generations. The time to act is now.”

Phelps is sworn in during a Oversight and Investigations hearing (Getty)

The U.S. government provides funding for the World Anti-Doping Agency budget and the House committee that Phelps addressed can make recommendations on any funding increases they feel necessary to improve the current system.

But that wasn’t the true purpose of Phelps’ appearance, who spoke alongside the U.S. Anti-Doping Agency (Usada) chief executive Travis Tygart. Instead, the Usada hope his enormous public profile will help to apply extra leverage on the IOC to change tact in the fight against drugs cheats, having been criticised for their soft treatment of Russia ahead of Rio 2016.

Tygart wants to see Wada made independent from the IOC (Getty)

Tygart expressed these frustrations: by ignoring calls to ban Russia from the Rio Olympics and leaving the decision to the 28 individual sport governing bodies, he said, the IOC had proven itself to be toothless and unwilling to get touch on state-sponsored doping. He also voiced the opinion that Wada should be made independent of the IOC.

"At least two Olympic Games were corrupted and at the Rio Games this past August, scores of Russian athletes were allowed to compete without credible anti-doping measures," Tygart added.

"When the moment came, despite mountains of evidence and vocal opposition from anti-doping leaders and clean athletes from around the world, the IOC chose to welcome the Russian Olympic Committee to Rio."

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IOC President Thomas Bach had, unsurprisingly, declined an invitation to attend. But the hearing was further evidence that there is a renewed commitment from American legislators to pressurise the IOC into action.

After the hearing swimming coach Bowman had a message for his former charge. “I have never been prouder of Michael Phelps than I am today,” he tweeted. “Well done.”