Shane Warne's international cricket career hangs in the balance after he failed a drugs test ahead of the World Cup.

Warne denied taking performance enhancing drugs and said he had unknowingly taken a banned diuretic.

Warne had simply taken a pill given to him by his mother in circumstances that were "totally innocent", a source close to his family told AAP.

He was due back to fly back into Melbourne on Tuesday night and will face an Australian Cricket Board anti-doping panel later this week.

The World Cup technical committee will consider an Australian request for a replacement player as no offence took place at the tournament.

The International Cricket Council's penalty for a positive drug test for diuretics is a two year ban, unless there is proof of mitigating circumstances.

Warne's departure sent shockwaves through the World Cup, with his announcement coming just half an hour before Australia's opening game of its title defence, against Pakistan.

A two year ban could end Warne's career which has yielded 491 Test wickets an average of 25.71 from 107 matches and 291 limited overs wickets at 25.82 in 193 matches.

The ACB said if Warne was cleared, it would seek permission for him to return to South Africa to play out the World Cup.

ACB chairman James Sutherland said he would also ask the World Cup technical events committee to allow a replacement to be brought into Australia's squad in the interim.

Warne, 33, said he was told on Monday he had failed a January 22 drugs test in Sydney because of a diuretic he took before his comeback match from a dislocated shoulder.

Diuretics are used mainly to rid the body of fluids and aid weight loss, but can also be used to mask other performance enhancing drugs.

Warne has been on a fitness regime for over a year, trimming about 14kg from his previously stout frame.

Warne, in a statement to reporters in Johannesburg, said he took a fluid tablet without knowing it contained a substance on the International Cricket Council (ICC) banned list.

"I was shocked and absolutely devastated to be informed by ASDA (Australian Sports Drug Agency) yesterday that a test sample which was collected in Australia on the 22nd of January indicated the presence of a prohibited substance," said Warne on Tuesday.

"The full process of analysing and testing procedures are not complete until my B sample is fully examined later this week in Australia.

"I'm shocked because I do not take performance enhancing drugs and never have and do not condone them in any way, shape or form.

"I'm proud to be in the shape I'm in at the moment and that is due to nothing other than hard work and looking after myself with diet.

"I did take a fluid tablet before my comeback game in Sydney which I did not know contained a prohibited substance.

"The tablet actually dehydrates you and gets rid of any excess fluid in your body and, as I understand it, is not performance-enhancing in any way."

Warne said he had decided to return home in the best interests of the team in their World Cup campaign and to address the situation personally.

"I have never taken any performance-enhancing drugs and am confident this matter will be cleared up very soon.

"My previous tests have always come back negative and so will any future ones."

Australian players looked stunned as they walked to the team bus before taking on the Pakistanis at the Wanderers stadium.

"I've spoken to my team-mates and they are very supportive," said Warne.

"I wish them luck for the rest of the tournament.

"Whether I play a part or not will be decided shortly.

"Until then I will just have to deal with the situation as best I can."

Warne thanked the World Cup squad for their support and friendship.

"I believe they have the talent and spirit in the team to win the World Cup, with or without me."

Sutherland praised Warne for informing the board about his test as soon as he heard from the Australian Sports Drug Agency.

"The results are preliminary and subject to confirmation," said Sutherland, referring to Warne's B sample.

"We support ASDA in its random testing ... at the same time we commend Shane for his open and co-operative approach, including coming forward as soon as the information was raised with him rather than following the normal process of waiting for ASDA to conduct and finalise further tests.

"If the anti-doping committee finds that Shane has not committed an offence under the ACBs policy, we will ask the World Cup events technical committee for approval for Shane to be reinstated to the World Cup squad."

Meanwhile Australia would apply for approval to replace Warne and had asked selectors to pick a replacement.

World Cup executive director Ali Bacher said: "Obviously Shane has been a great leg spin bowler, probably the greatest of all time and obviously we want the best at the World Cup. But rules are rules and you have to abide by them."

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