Čavić, who lost to Phelps by just one hundredth of a second in the 100 metres butterfly finals at the Beijing 2008 Olympics, also claimed "doping is getting worse" in the sport.

It followed Phelps' appearance at a Congressional Sub-Committee in Washington D.C. last week in which he claimed he never competed against a totally clean field during his career.

American Phelps, winner of 23 Olympic gold medals and the most decorated Olympian, also claimed that the United States has better anti-doping standards than other parts of the world.

But this view has been criticised by Čavić, who cited the example of disgraced US cyclist Lance Armstrong to illustrate how passing drugs tests is not necessarily the sign of a clean athlete.

He then described Phelps' recovery rate as "nothing short of science fiction" before targeting an alleged reluctance to previously support the athlete biological passport system.

"Doping has been a problem and it's only getting worse," the Serbian wrote in an open letter posted on Twitter and addressed to Phelps.

"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.

"I could recall Lance Armstrong getting tested three times 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 hope that you stick with this because in case our sons go pro some day, I'd like to think you made a difference."

This is not the first time that Čavić and Phelps have publicly clashed.

Čavić, who is based in California, also claimed before their Beijing clash that it would be "good for the sport" if Phelps lost before repeatedly insisting that it was his "gut instinct" that he had touched first in the 100m butterfly.

He also attempted to trash talk his rival before finishing second behind him again at the 2009 World Championships in Rome.

Phelps has not yet replied to these latest comments.