Gwangju - Australian relay world-record holder Shayna Jack denied wrongdoing on Saturday after she failed a drugs test and was forced to miss the swimming world championships in South Korea.

The 20-year-old, part of Australia's 4x100m freestyle team that set a world record at last year's Commonwealth Games, failed an out-of-competition test late last month and has been suspended, Swimming Australia said.

Jack, who returned home days before the start of the world championships in South Korea citing "personal reasons", denied deliberately taking a banned substance.

The positive test will cause severe embarrassment to Australian swimming, however, after Olympic champion Mack Horton's protest against Chinese rival Sun Yang in Gwangju over salacious claims he smashed vials of blood following a test last year.

"I did not take this substance knowingly," Jack posted on Instagram.

"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 or jeopardise my career."

Australian officials ushered Jack back to Australia from a world championship training camp in Japan after the results of the June 26 test and gave her a provisional suspension, Swimming Australia said.

"Swimming Australia is and always has been committed to a clean sport," CEO Leigh Russell said in a statement.

"We have consistently supported a zero tolerance approach to doping and have been strong supporters of a rigorous and consistent approach to drug testing both here and overseas."

Australia's Horton took a highly visible stance when he snubbed Sun at a medals ceremony, provoking a furious reaction from China's 11-time world champion, who accused him of "disrespecting China" by not stepping on the podium.

Britain's Duncan Scott copied Horton after Sun retained his 200m freestyle title and several swimmers, including Olympic champions Adam Peaty and Lilly King, spoke out in support of the two.

But Sun's coach Denis Cotterell slammed his fellow Australians for "double standards", noting Horton's Australian team-mate Thomas Fraser-Holmes was banned by FINA for 12 months for missing a trio of drug tests.

Jack's failed test will now supply Sun and China with plenty of ammunition to shoot back.

"As you would expect, we are bitterly disappointed with allegations a swimmer has a prohibited substance in her system although it is important to point out that the matter is yet to be determined," said Russell. 

"We will continue to provide appropriate support for Shayna. We will also provide support for our team members who are still in Korea and our team and our organisation will continue to reaffirm our zero tolerance approach."

View this post on Instagram

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

A post shared by Shayna Jack (@shayna_jack) on Jul 27, 2019 at 1:06am PDT