Former tennis world number one Maria Sharapova said on Monday that she recently received a letter saying she had failed a drug test at the Australian Open.

The 28-year-old Russian, a five-times grand slam champion, has not competed since she lost to Serena Williams in the quarter-finals of the Australian Open in January and has struggled with a series of injuries in recent years.

Sharapova said she tested positive for meldonium and that she did not look at a list of banned substances for 2016 that the World Anti-Doping Agency had sent last December.

"I was first given the substance back in 2006. I had several health issues going on at the time," Sharapova told a news conference in Los Angeles.

"I was getting sick very often … and I had a deficiency in magnesium and a family history of diabetes, and there were signs of diabetes. That is one of the medications, along with others, that I received."