Australian Olympian Peter Bol has been provisionally suspended after failing an out-of-competition drug test.
Bol, who finished fourth in the 800 metres at the Tokyo Olympics, returned a positive result for the banned substance EPO.
Bol's suspension has been backdated to January 10, meaning he cannot train or compete at any level.
He has declared his innocence in a statement posted to his Twitter account.
"It is critically important to convey with the strongest conviction I am innocent and have not taken this substance as I am accused," the statement read.
"I ask that everyone in Australia believe me and let the process play out."
Bol said he was in "total shock" when he found out last week his urine sample taken in October had come back positive for synthetic EPO I.
"To be clear, I have never in my life purchased, researched, possessed, administered or used synthetic EPO or any other prohibited substance.
"My career hopes and dreams are literally hanging in the balance over these next few weeks."
Athletics Australia CEO Peter Bromley said the test result was "extremely concerning".
"There are procedural fairness and investigative considerations that constrain how much we can say, and at this point it would be inappropriate for Athletics Australia or anyone else to speculate about the specific details or pre-empt any outcome," he said.
"However, what we can say is that learning about this adverse analytical finding was both extremely concerning and completely out of the blue, and we will support Sport Integrity Australia, who are leading the investigation into the matter.
"Our primary consideration right now is that the appropriate process is followed and that it is not undermined by inappropriate speculation.
"The welfare of our athletes remains critical through this process, and we will continue to do all that is appropriate to ensure both Peter and other athletes, coaches and support staff are provided with the necessary support."
Bol's manager, James Templeton, told the ABC he and the runner would not comment further on the matter, while his coach Justin Rinaldi took to Twitter to defend him.
"I can 100 per cent confirm that Pete has NEVER even considered taking a performance enhancing drug, let alone inject himself with one," he wrote.
"We have always been open & transparent in regards to our training and will continue to do so. All we can do is trust in the process and be open & honest."
Bol, Western Australia's Young Australian of the Year, had been due in Canberra next week for the announcement of the Young Australian of the Year.
The 28-year-old, who was born in Sudan and fled the civil war with his family aged four, won Australian fans over with his gutsy performance at the Tokyo Olympics, before taking silver at the 2022 Commonwealth Games in Birmingham.
