Sharapova, 28, tested positive for meldonium, a substance she has been taking since 2006 for health issues.

The five-time Grand Slam champion is provisionally suspended from 12 March pending further action.

The sportswear company, Nike, announced that it was halting its relationship with the player until the investigation is complete.

"I did fail the test and take full responsibility for it," said Sharapova, who won the Wimbledon title as a 17-year-old in 2004.

"For the past 10 years I have been given a medicine called mildronate by my family doctor and a few days ago after I received a letter from the ITF [International Tennis Federation] I found out it also has another name of meldonium, which I did not know."

Sharapova's lawyer, John Haggerty, told Sports Illustrated he was attempting to speak to the ITF because "we think there is a laundry list of extremely mitigating circumstances that once taken into consideration would result in dramatically reducing any sanction that they might want to impose on Maria".

Sharapova, who lives in Florida, provided the anti-doping sample in question on 26 January, the day she lost to Serena Williams in the Australian Open quarter-finals.

The World Anti-Doping Agency (Wada) analysed the sample and returned a positive for meldonium, leading to the Russian being charged on 2 March.

"It is very important for you to understand that for 10 years this medicine was not on Wada's banned list and I had been legally taking that medicine for the past 10 years," said Sharapova.

"But on 1 January the rules had changed and meldonium became a prohibited substance, which I had not known."

She added: "I received an email on 22 December from Wada about the changes happening to the banned list and you can see prohibited items - and I didn't click on that link."

(Extracts from BBC)