Morocco's Mariem Alaoui Selsouli, one of the favourites for the Olympic women's 1500m gold medal, failed a drugs test and has been ruled out of the Games, the world athletics body said Wednesday.
The 28-year-old, silver medallist at the World Indoor Championships in Istanbul in March, tested positive for a banned diuretic.
She failed the test after taking part in the Paris Diamond League meeting on July 6, where she had set the world's best time this year of 3min 56.15sec.
Selsouli, who may have also run the 5,000m in London, served a two-year suspension from 2009 to 2011 for an earlier doping offence and now faces a lifetime ban.
"The prohibited substance furosemide was found," the International Association of Athletics Federations (IAAF) said in a statement.
"Alaoui Selsouli has waived her right to the B analysis, and as a result has now been provisionally suspended from all competition in athletics.
"In accordance with IAAF rules, she still has the right to request a hearing which needs to be held by the Moroccan Athletic Federation within the next three months."
Selsouli started the outdoor season with a personal best of 8min 34.47sec in the 3,000m in Eugene, Oregon in June. Six days earlier, she had won the 5,000m in Rabat in a time of 14min 45.91sec.