Former world number one Maria Sharapova has admitted to the “huge mistake” of failing a drug test at this year’s Australian Open grand slam.

The tennis star made the announcement at a press conference in Los Angeles on Tuesday morning (AEDT), which came as a shock to the journalists in the room as she was widely expected to announce her retirement.

Sharapova said she was unaware that the drug meldonium, which she said was prescribed to her by a doctor for “several medical conditions”, had been banned as of 1 January 2016.

What is meldonium? Click the owl for full details.  

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“I failed the test and take full responsibility for that,” she said.

“It is important for you to understand that for 10 years this substance was not on WADA’s banned list.

Sharapova denied knowing meldonium was banned. Photo: Getty

“I received a letter on 22 December from [the World Anti-Doping Agency] for the changes next year and where the tests will be with a link to the changes for 2016 and I did not look at that list.”

The BBC reported after the press conference that Sharapova was provisionally suspended from 12 March.

She reportedly provided a sample on 26 January after losing the Australian Open quarter final to Serena Williams.

WADA determined the sample was positive for meldonium and charged her on 2 March, according to the BBC.

Meldonium is usually prescribed by a doctor to treat reduced blood flow to the heart, such as for sufferers of angina and myocardial infarction.

The substance was banned because it is thought to increase endurance, improve rehabilitation, protect against stress and enhance the functions of the central nervous system, as reported by a journal article in Drug Testing and Analysis.

The only hint from Sharapova’s management company IMG before the press conference was that her announcement would be “very big news”, prompting speculation she would retire.

Sharapova provided the sample after losing to Williams in January. Photo: Getty

“I know many of you thought I’d be announcing my retirement. If I was ever going to announce my retirement, it would not be in a downtown Los Angeles hotel with a fairly ugly carpet,” she told the media.

A range of injuries have interrupted Sharapova’s career. The 28-year-old has not picked up a racquet since losing to Serena Williams in the quarter-finals of this year’s Australian Open. She has played just three tournaments in the past eight months, and has suffered a succession of problems with her right arm and shoulder.

Despite these injuries, and the possibility of sanctions, Sharapova said she hoped to play again.

“I let my fans down, I let the sport down that I’ve been playing since the age of four, that I have loved so deeply.

“I know with this I face consequences and I don’t want to end my career this way, and I really hope I get another chance to play this game.”

Despite not playing since January, Sharapova is still ranked seventh in the world after a glittering career that has seen her win five grand slams and 35 WTA titles.