Swimming Australia (SA) has confirmed rising star Shayna Jack tested positive to a banned substance, which forced the 20-year-old to withdraw from the national squad before the world championships being held in Gwangju, South Korea.

Key points: Shayna Jack failed a routine out-of-competition doping test in late June

Shayna Jack failed a routine out-of-competition doping test in late June Jack says she did not take the banned substance knowingly

Jack says she did not take the banned substance knowingly Her manager says Jack is "not coping very well"

After initially claiming Jack had left the team before the world titles for "personal reasons", SA said she had failed a routine out-of-competition drug test in late June.

"Swimming Australia tonight confirmed that swimmer Shayna Jack has been notified by ASADA of an adverse test result following a routine out-of-competition drug test conducted by ASADA testers on June 26, 2019," an SA statement said.

"Once Swimming Australia was made aware of the adverse test result, it immediately took action — in accordance with the national policy — to provisionally suspend Shayna from the Australian swim team while a process was under way and accompanied her back to Australia from a training camp being held in Japan.

"The Swimming Australia policy also means that any Australian athlete under provisional suspension, while ASADA investigations are under way, cannot take part in any competition, meaning Shayna was unable to travel to Gwangju to compete at the 2019 World Championships."

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Jack took to Instagram to explain why she left the Australian squad ahead of the world titles.

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

"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."

Jack's manager Philip Stoneman says it is "shattering" for his client.

"She's not coping very well at all. She's feeling incredibly vulnerable. She's scared," he said.

"She understands that people are going to make an opinion of her and that's something that she can't control and it's going to be … there's just nothing she can do about that. She's not a cheat and all of this is happening at a time when the meet is still going.

"She wants to be focussed on the Australian athletes, not on what's happened to her. So she's feeling very bad for them. It couldn't be any worse for her … as a 20-year-old, you can imagine."

Mr Stoneman said Jack was aware of what the banned substance was that led to the positive test.

"She knows what it is. We haven't actually discussed that," he said.

"She's been told what it is — she doesn't understand what it is but she's been told."

Jack had initially claimed she withdrew from the world titles because of "personal reasons". ( AAP: Matt Roberts )

Jack's Australian teammate Cate Campbell said the national squad in Gwangju had been unaware of the positive test.

"I had absolutely no knowledge of this before tonight," Campbell said.

"All I knew before is that it was a personal matter and we were respecting her privacy.

"I think that we have to respect the process. We stand for clean sport and I think the fact that Shayna isn't here at the moment strengthens that stance."

Former ASADA chief executive Richard Ings was among those critical of how Jack's positive test was handled by both the swimmer and SA, when "personal reasons" was offered as the explanation for her withdrawal.

"We now know this was an untruth," Ings tweeted.

"The real reason, known at the time of this announcement, was she had been provisionally suspended for a positive A sample drug test. Athletes need to be frank from day one."

Failed test follows Sun Yang controversy at world titles

Jack's positive test comes at a time when Australia has been front and centre at the world championships amid the ongoing campaign to clean up swimming and improve anti-doping procedures.

Australian swimmer Mack Horton sparked controversy when he refused to acknowledge Sun Yang after the Chinese star relegated him to 400 metres freestyle silver in Gwangju on Sunday.

Sun, who served a doping ban in 2014, faces a lifetime suspension if found guilty of charges that he smashed a vial of his blood with a hammer in a clash with testers at his home last year.

The allegations of doping rule violations could result in a ban from the 2020 Tokyo Olympics and he has requested a public trial at the Court of Arbitration for Sport (CAS) in September to defend himself.

Sun Yang (centre) holds up his gold medal, as Mack Horton (left) stands away from the podium. ( AP: Mark Schiefelbein )

Horton refused to share the medal podium with Sun, choosing to stand behind it, while also declining to be photographed with the gold medallist.

Swimmers quickly threw their support behind Horton, who was angry that Sun had been allowed to compete ahead of the CAS hearing.

Mr Stoneman said Jack had been "fully supportive" of where Horton stood in relation to doping in swimming.

"She doesn't want to see cheats in the sport," he said.

"It's as simple as that and that's what's making this so much more difficult for her to deal with because she's now going to have the finger pointed at her for being exactly that."

Two-time Canadian Olympian and human rights lawyer Nikki Dryden said Sun might feel as though he would avoid further scrutiny from Australia followed Jack's positive test.

"If I was Sun Yang I would feel pretty vindicated," she said.

Jack could miss Tokyo Olympics

Ms Dryden said Jack was "innocent until proven guilty", however she believed the young swimmer would not be available for selection for next year's Tokyo Olympics because of the potential length of an ASADA investigation.

"ASADA could continue to investigate for months and months and months," she said.

"Athletes are suspended for a whole period of time while it's happening. In the short term, her Olympics are gone, probably. There's no way it could be concluded before [the] Olympic trials in time for her to go and compete in Tokyo."

Shayna Jack (left) with her Australian relay teammates after setting a world record at the Commonwealth Games. ( AAP: Darren England )

Ms Dryden said Jack would also face a significant financial burden in the effort to clear her name.

"The cost to defend herself is going to be huge," she said.

"She'll incur debt, whether she is clean or dirty, whereas the swimming federations and ASADA have all of the resources. So it will be a very uneven fight."

Jack was a member of Australia's 4x100m freestyle relay team that set a world record at last year's Commonwealth Games on the Gold Coast.

She also won two silver and two bronze medals in relays at the 2017 world championships in Budapest.

ABC/AAP