Russian tennis superstar Maria Sharapova has admitted she failed a drugs test at this year's Australian Open.
She tested positive for meldonium, the heart attack drug only added to the World Anti-Doping Agency (WADA) banned list on January 1.
Sharapova claims not to have read the list of recently banned substances.
"I made a huge mistake and I let my fans down," she said when making the announcement here this afternoon.
"I failed the test and take full responsibility for that.
"I had been taking this medicine for the past 10 years, but on January 1 this became a prohibited substance which I did not know.
"I let the sport down."
Sharapova claimed she was prescribed the medication for health issues dating back to 2006, including magnesium deficiency, an irregular heartbeat, and a family history of diabetes.
"It's very important for you to understand that for 10 years, this medicine was not on WADA's ban list, and I had been legally taking the medicine for the past 10 years," she said.
"Throughout my long career, I have been very open and honest about many things, and I take great responsibility and professionalism in my job every single day, and I made a huge mistake."
The 28-year-old, currently ranked seventh in the world, has not competed since she lost to American rival Serena Williams in the quarter-finals of the Australian Open at the Rod Laver Arena in Melbourne in January.
She tested positive following that defeat on January 26.
"As meldonium is a non-specified substance under the WADA (and, therefore Tennis Anti-Doping Programme) list of Prohibited Substances and Prohibited Methods, Ms Sharapova will be provisionally suspended with effect from March 12, pending determination of the case," said an International Tennis Federation (ITF) statement.
The news comes as a major shock for the tennis world after news of her impending "major announcement" yesterday triggered speculation the five-time Grand Slam champion was going to announce her retirement.
Sharapova, winner of the Olympic silver medal in the singles at London 2012, started playing tennis in Sochi when she was two but the family moved to Florida when she was seven.
She has lived in the United States ever since.
"I don't want to end my career this way," Sharapova added.
"I know many of you thought I was retiring.
"But if I was ever going to announce my retirement it would not be in this downtown Los Angeles hotel with this fairly ugly carpet."
Russian Tennis Federation President Shamil Tarpishchev has already leapt to Sharapova's defence.
"I think that it’s nonsense," the International Olympic Committee (IOC) member told Russian news agency TASS.
"Athletes take what their physiotherapists advise them.
"I believe that Sharapova will still have a chance to play at the  Olympics, though we will see how things are going to develop."
Steve Simon, chief executive of the Women's Tour Association, was sympathetic but less confident Sharapova would avoid punishment.
"I am very saddened to hear this news about Maria," he said.
"Maria is a leader and I have always known her to be a woman of great integrity.
"Nevertheless, as Maria acknowledged, it is every player's responsibility to know what they put in their body and to know if it is permissible.
Four countries and four Olympic sports have now suffered doping failures for meldonium in recent weeks.
It is expected many more will follow.
The list of leading names to have tested positive for the drug so far include Ethiopia's Tokyo Marathon winner Endeshaw Negesse and Sweden's Ethiopian-born former world 1500 metres champion Abeba Aregawi.
Ukrainian biathletes Artem Tyshchenko and Olga Abramova have also failed for the substance, a metabolic enhancer which can increase endurance.
Russian cyclist Eduard Vorganov also failed for the drug last month
The heart attack drug was moved from the monitored to the prohibited list by WADA due to “evidence of its use by athletes with the intention of enhancing performance”.
"WADA is aware of the ongoing case," they said in a statement.
"As is our normal process, and in order to protect the integrity of the case, WADA will refrain from commenting further until a decision has been issued by the ITF.
"Following that, WADA will review the reasons for the decision and subsequently decide whether or not to use its independent right of appeal to the Court of Arbitration for Sport."