Former world number one Maria Sharapova has revealed she failed a drugs test at the Australian Open.

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".

Test failed on day of Williams defeat

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."

'I made a huge mistake'

Sharapova has been the highest-earning female athlete in the world in each of the past 11 years, according to the Forbes list. Her career earnings from tennis alone amount to almost £26m.

She first reached world number one in August 2005 and is currently seventh in the rankings - but she has played just four tournaments since Wimbledon last July as she struggled with an arm injury.

Nike, which has long sponsored her, released a statement early on Tuesday saying: ""We are saddened and surprised by the news about Maria Sharapova.

"We have decided to suspend our relationship with Maria while the investigation continues."

Sharapova, who turns 29 in April, hopes to be able to return to tennis in the future.

"I made a huge mistake," she said. "I have let my fans down, and let the sport down that I have been playing since the age of four that I love so deeply.

"I know that with this I face consequences and I don't want to end my career this way. I really hope to be given another chance to play this game."

What the Sharapova camp is saying

Haggerty said Sharapova started to take meldonium after her doctor did "an extensive battery of tests to determine what medical conditions were causing her to be sick on a frequent basis".

She had "abnormal electrocardiogram readings" and "some diabetes indicators", which prompted the doctor to recommend medication, including meldonium.

He added: "She took it on a regular basis as recommended by her doctor. He told her what to take and when to take it, and then continued to test her and confirm that it was giving her the desired improved medical condition."

Wada placed meldonium on its monitoring programme in 2015 before adding it to the banned list this year "because of evidence of its use by athletes with the intention of enhancing performance".

Haggerty said: "Regrettably, when they added this to the ban list on 1 January of this year, she did not pick that up."

What happens now?

Sharapova could apply for a retroactive therapeutic use exemption (TUE).

A TUE allows a player to use a banned substance, without committing an anti-doping rule violation, if they have a medical condition that requires it.

Sharapova's lawyer Haggerty added that she has waived the testing of a B sample.

How long could she be banned for?

Up to four years, according to Tennis Anti-Doping Programme and Wada guidelines.

But Jeff Tarango, Sharapova's former coach and an ex-Tour professional, said he doubted she would be banned for that long.

"I think it immediately falls under two years but with these circumstances probably one year," the American told BBC Radio 5 live.

"She can apply for a TUE. If it really is something she had to take for her heart and diabetes then it falls under a TUE.

"If it is something where her heart in 120C weather would just give out without taking this then I'd rather she took it."

Has anyone else been suspended for a doping violation?

Several high-profile Tour players have been suspended for anti-doping violations - including Marin Cilic, Viktor Troicki and Barbora Strycova, who all had the length of their bans later reduced.

However, each of these offences were committed under different circumstances to Sharapova's case.

The ban for 2014 US Open champion Cilic, for taking a glucose tablet bought at a pharmacy in France, was reduced from nine to four months in October 2013.

Troicki was suspended for 12 months on appeal after refusing to take a blood test at a tournament in Monte Carlo in 2013, claiming he was feeling unwell and had a phobia of needles.

Also in 2013, Czech player Strycova was given a back-dated six-month ban after saying a banned stimulant entered her system via a weight-loss supplement.

Former Grand Slam winners Martina Hingis and Andre Agassi are among those who have been previously banned for testing positive for recreational drugs.

Was this news a surprise?

Completely. There had been speculation Sharapova was going to announce her retirement at a Los Angeles news conference, which was streamed live online.

"I know many of you thought that I would be retiring today but if I was ever going to announce my retirement it would not be in a downtown Los Angeles hotel with this fairly ugly carpet," she said.

What is meldonium?

It is meant for angina patients but athletes like it because it helps their endurance and ability to recover from big efforts.

It is on the banned list now because Wada started seeing it in lots of samples and found it does have performance-enhancing properties. It was then added to Wada's 'watchlist' for over a year. The decision to ban it was communicated last autumn and it was added to the banned list on 1 January.

Made in Latvia, it is widely available - without prescription and at low cost - in many east European countries but it is not licensed in most western countries, including the United States.

It is widely thought that hundreds of athletes have been using it and there are a lot more cases in the pipeline.

'Hammer blow to the sport'

BBC tennis commentator Andrew Castle said: "After the betting revelations, this Sharapova news is a hammer blow to the sport.

"Wada issue their list of banned substances and you don't look? Nor do management?"

Without naming Sharapova, three-time Grand Slam singles champion Jennifer Capriati said she was "extremely angry and disappointed" and "never opted to cheat no matter what".

The American, whose own career was ended by injury, added: "If this medication helped me to come back again would everyone be all right with me taking it?

"In my opinion, if it's all true every title should be stripped. This is other people's lives as well."

Women's Tennis Association (WTA) president Steve Simon said he is "very saddened" at Sharapova's failed test.

"Maria is a leader and I have always known her to be a woman of great integrity," he added.

"As Maria acknowledged, it is every player's responsibility to know what they put in their body and to know if it is permissible.

"This matter is now in the hands of the Tennis Anti-Doping Programme and its standard procedures. The WTA will support the decisions reached through this process."