Morocco's Mariem Alaoui Selsouli, one of the favourites for the Olympics 1500m gold medal, has failed a drugs test and has been ruled out of the Games, a source with knowledge of the case said Monday.

The 28-year-old tested positive for a banned diuretic at the start of July, the source said, confirming a report carried by sports daily L'Equipe.

Selsouli 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 might have also taken part in the 5,000m in London, served a two-year doping ban from 2009 to 2011 for an earlier doping offence.

The silver medallist at the world indoor championships in March in Istanbul, she started the outdoor season with a personal best of 8min 34.47sec in the 3000m in Eugene, Oregon in June.

Six days earlier, she had won the 5,000m in Rabat in a time of 14min 45.91sec.