Australia's swimmers could be stripped of their 4x100m medley relay bronze medals from the London 2012 Olympics after breaststroker Brenton Rickard failed a re-test of his eight-year-old doping samples, a report said Friday.

Rickard, 37, described the positive test for a small amount of furosemide, a masking agent, as his "worst nightmare" in an email to his former team-mates published by the Sydney Morning Herald.

"I have always abhorred doping within the sport so you can imagine how sickened and horrified I am to find myself in this predicament. This is my worst nightmare," the email was quoted as saying.

If the result is confirmed, the entire Australian relay team -- Rickard, James Magnussen, Christian Sprenger, Hayden Stoeckel, Matt Targett and Tommaso D'Orsogna –- would lose their medals.

Australia has never lost an Olympic medal over a doping violation, the Herald said.

Rickard said the case will be heard on Monday at the Court of Arbitration for Sport in Lausanne, Switzerland. 

Rickard, who swam the heat in London but not the final, also won two silver medals at Beijing 2008 along with two long-course world titles, and once held the 100m breaststroke world record.

The 2009 Australian athlete of the year said he believed the furosemide, a banned diuretic, came from contaminated over-the-counter medications.

"Unfortunately for me though, I am not able to produce physical evidence eight years after the fact," he wrote.

The report comes after Australia's Shayna Jack tested positive for a proscribed muscle-building drug in the run-up to the 2019 world championships, and was later banned by the Australian anti-doping authority.