LOS ANGELES: Russian tennis star Maria Sharapova, the highest-paid woman in sports, said on Monday that she failed a drug test at the Australian Open due to a substance she has been taking for 10 years for health issues.

The 28-year-old Sharapova, a five-time grand slam champion, will be provisionally suspended starting March 12, the International Tennis Federation (ITF) said.

She is the seventh athlete in a month to test positive for meldonium, which is used to treat diabetes and low magnesium, and was only banned by the World Anti-Doping Agency as of January 1.

"I made a huge mistake. I let my fans down and I let the sport down," said Sharapova, a teenage tennis prodigy who became the third-youngest Wimbledon champion. "I take full responsibility for it."

"I know that with this I face consequences and I don't want to end my career this way. I really hope that I will be given another chance to play this game," former world No.1 Sharapova told a news conference in a downtown Los Angeles hotel.

Hours after the announcement, one of her biggest sponsors, Nike Inc., said it had suspended its relationship with her while the investigation continues, cable news channel CNBC reported.

Officials at Nike, the world's largest sportswear maker, were not immediately available for comment.

"We are saddened and surprised by the news about Maria Sharapova," CNBC quoted a Nike statement as saying.

The ITF's anti-doping programme calls for a four-year suspension for a positive test, but that ban can be reduced in various circumstances, such as for first-time offences or if the player shows no significant fault or negligence. If a player bears no fault or negligence, there is no suspension.

According to Forbes, she earned $29.5 million in 2015, mostly from endorsements.

Sharapova said her family doctor had been giving her mildronate, which is also called meldonium, for 10 years after she frequently became sick, had irregular EKG results, a magnesium deficiency and a family history of diabetes.

"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 the medicine. But on January the first, the rules have changed and meldonium became a prohibited substance."

The World Anti-Doping Agency, or WADA, declined to comment until ITF issues a final decision.

Meldonium is used to treat chest pain and heart attacks among other conditions, but some researchers have linked it to increased athletic performance and endurance. It is listed by WADA among its prohibited metabolic modulators, along with insulin, and some researchers say it can also help recovery.

It is not approved in the United States but is available in Russia, Latvia and other countries in that region. Over the past month, Russian cyclist Eduard Vorganov, Russian figure skater Ekaterina Bobrova and Ethiopia-born athletes Endeshaw Negesse and Abeba Aregawi and Ukraine biathletes Olga Abramova and Artem Tyshchenko have all tested positive for meldonium.

Sharapova is the most prominent tennis player to test positive for a banned substance in recent years.

Croatia's Marin Cilic was banned for nine months in 2013 after testing positive for a prohibited stimulant, though the suspension was cut to four months on appeal.

Former No. 1 Swiss player Martina Hingis retired after receiving a two-year suspension for a positive cocaine test in 2007, though she denied taking the drug.

Last year, the sport banned US player Wayne Odesnik for 15 years after his second doping violation, testing positive for steroids and other banned substances.

Sharapova is the biggest name in sport to test positive since New York Yankees baseball slugger Alex Rodriguez was banned for a year in 2013 after using performance-enhancing drugs and American cyclist Lance Armstrong was banned for life from racing in 2012 after a US Anti-Doping investigation.

Sharapova, one of the most popular figures in global sports, has long been a favourite with her sponsors. Cosmetics maker Avon Products Inc declined to comment on its endorsements.

Steve Simon, CEO of the Women's Tennis Association, said in a statement he was saddened to hear the news.

"Maria Sharapova is a leader and I have always known her to be a woman of great integrity," he said. "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. The WTA will support the decisions reached through this process."

The news came a day after Sharapova's management team said she was going to make a "major announcement," which had many speculating that she was going to announce her retirement from professional tennis.

Maria Sharapova, who has struggled with a series of injuries in recent years, has not competed since she lost to Serena Williams in the quarter-finals of the Australian Open in January.

Renowned for her never-say-die approach, a gritty baseline game and high-decibel shrieking, Sharapova at 17 became the first Russian woman to win Wimbledon when she beat Serena Williams 6-1 6-4 in the 2004 final.

That victory also made her the third-youngest Wimbledon champion, behind only Lottie Dod and Martina Hingis, and the fourth-youngest grand slam winner in the open era after Hingis, Monica Seles and Tracy Austin.