Maria Sharapova has failed a drug test from the Australian Open Tennis Championships 2016 and has been provisionally suspended.
The five-time Grand Slam winner made the shock announcement at a press conference in Los Angeles, saying the list of banned substances had changed and she wasn't aware she'd broken the rules.
Sharapova, 28, said she'd taken the drug, named Meldonium usually used to treat heart conditions, for 10 years, as prescribed by her family doctor but she received a letter this week from the International Tennis Federation saying it had become a prohibited substance.
"I failed a test and I take full responsibility for it," Sharapova said.
"It is very important for you to understand that for 10 years this medicine was not on [the World Anti-Doping Agency's] banned list and I had been legally taking the medicine for the past 10 years, but on January 1st, the rules had changed and Meldonium became a prohibited substance which I had not known."
The International Tennis Federation released a statement shortly after the press conference saying "Sharapova will be provisionally suspended with effect from 12 March, pending determination of the case".
It's not yet known how long the suspension will last.
An emotional Sharapova said she took responsibility for her actions.
"I made a huge mistake. I have let my fans down, I have let the sport down that I have been playing since the age of 4, that I love so deeply," Sharapova said.
"I know that with this I face consequences and I don't want to end my career this way."
Sharapova said she'd been prescribed Meldonium in 2006 to treat several health issues including a deficiency in magnesium, irregular electrocardiogram heart results and a "family history of diabetes".
"It made me healthy and that's why I continued to take it," Sharapova said.
Clinical uses of Meldonium include the treatment of heart disorders including chronic heart failure, myocardial infarction or heart attack as well as pain condition angina.
It is not approved for clinical use by the U.S. Food and Drug Administration and is manufactured in Latvia by pharmaceutical company Grindeks.
Meldonium also has a metabolic enhancement effect, which is why it was banned by the U.S. Anti-Doping Agency in 2015 citing "the abuse of this substance". The World Anti-Doping Agency first monitored the drug in 2015, and banned it on January 1, 2016.
The Australian Open was on January 16, 2016.
The federation confirmed Sharapova provided an anti-doping sample on January 26 at the 2016 Australian Open and was charged on March 2 with an Anti-Doping Rule Violation.
Two Russian cyclists have also tested positive to the drug this year.
Watch Sharapova's full conference here.
The news of the former world number one's suspension has shocked the tennis world, with the Women's Tennis Association chief executive officer Steve Simon saying: "Maria is a leader and I have always known her to be a woman of great integrity".
Former professional tennis player James Blake applauded Sharapova for facing the consequences with class.
Wow. Classy of @MariaSharapova to hold a press conference for this and admit making a mistake. Definitely agree that have to be aware though— James Blake (@JRBlake) March 7, 2016