Australian swimmer Shayna Jack has been banned for two years for a doping violation.

The Court of Arbitration for Sport (CAS) confirmed the result of a 2019 out-of-competition test showing Jack had ingested the banned substance Ligandrol, however the CAS said she did so accidentally.

The test became public when she was forced to withdraw from last year's World Swimming Championship in South Korea.

She was provisionally banned for four years, but the court found that because she did not deliberately take the drug, the penalty would be halved.

She will be free to return to competitive swimming next July, but that will be too late for her to qualify for the Olympics.

In a statement, the CAS said a sole arbitrator found "on the balance of probabilities, that Shayna Jack did not intentionally ingest Ligandrol and considered that she had discharged her onus of proving that the anti-doping rule violation was not intentional".

Jack always maintained that she had not knowingly taken the drug.

Writing on Instagram on Monday night, she said she was innocent.

"The CAS have confirmed in emphatic terms that I did not intentionally, knowingly or recklessly use Ligandrol in any manner," she wrote.

"I walk a little taller tonight with the fact that this ordeal is finally over."

"There was no evidence produced by my accusers as to how this substance entered my system."

"I have proven that I have NOT ever cheated, not used prohibited substances intentionally or knowingly."

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Prior to Jack's positive test, the Australian Sports Anti-Doping Authority had issued a warning that the number of athletes caught with the drug in their system had jumped from two in 2015 to nine in 2017.

Anti-doping experts warned about the danger of unknowingly taking banned drugs in contaminated supplements.

"The anti-doping rules are far from satisfactory and can produce results that are far from fair," Jack wrote on Instagram.

Jack said she would return to the sport.

"I am returning to swimming — the sport that I have loved all my life and the sport that I will cherish just that little bit more ongoing," Jack wrote.

The court's decision can be appealed by the World Anti-Doping Agency or world swimming's governing body, FINA.

Jack's case gained notoriety coming just days after Australian swimmer, Mack Horton, refused to stand on a podium in protest against Chinese Swimmer Sun Yang, who had been charged with an anti-doping violation but allowed to swim at the World Championships.

The Australian Sports Anti-Doping Authority, now Sports Integrity Australia, was also criticised in some circles for the length of time it took to make its case.