The head of the Russian Tennis Federation says Maria Sharapova's failed doping test could spell the end of her career.

Shamil Tarpishchev tells Russian news agency R-Sport that Sharapova's situation is "bad" and that it is "very doubtful" that she will resume her career.

Tarpishchev had previously predicted that Sharapova could return to competition in time to lead the Russian team at the Olympics in August. It was not immediately clear what prompted his change of view.

Sharapova tested positive for meldonium at the Australian Open in January. She said she had taken the drug for a decade for medical purposes and did not know it had been banned for 2016.