Aussie swimmer Shayna Jack has emphatically denied she’s a drug cheat and says she would never take a substance knowingly.

Jack, who withdrew from the world swimming championships before the event began for what was cited as personal reasons, tested positive to a banned substance.

Just hours after Swimming Australia confirmed the positive test from June 26, the 20-year-old took to Instagram to defend herself.

“They say a picture is worth a thousand words, in this case a picture can not describe the amount of pain and vulnerability I am feeling right now,” she said.

“It is with great sadness and heartache that I had to leave due to allegations of having a prohibited substance in my system.

“I did not take this substance knowingly.

“Swimming has been my passion since I was 10 years old and I would never intentionally take a banned substance that would disrespect my sport and jeopardise my career.”

Swimming Australia CEO Leigh Russell says the governing body is “bitterly disappointed” about Jack’s positive test while adding that the matter has yet to be determined.

Russell says the group will continue to provide appropriate support for Jack.

“There is an ongoing investigation and my team and I are doing everything we can to find out when and how this substance has come into contact with my body,” Jack posted.

“I would appreciate if you respect my privacy as this is very hard for me to cope with.”

The matter has cast a shadow over the Australian swim team, some of whom have been critical of FINA’s handling of doping matters.

Mack Horton snubbed Sun Yang on the medals podium after finishing second to the Chinese star in the 400-meter freestyle. Horton is angry that FINA allowed Sun to compete at worlds ahead of his hearing before the Court of Arbitration for Sport involving a clash with drug testers.