Moroccan 1500m runner Mariem Alaoui Selsouli has reportedly tested positive for a banned substance.
The 28-year-old is the favourite to take the gold in London, but now it looks unlikely that she will compete, according to French newspaper L'Equipe.
Selsouli served a two-year ban after testing positive for performance-enhancing drugs in 2009 and could now face a lifetime ban for a second offence.
She recently smashed her personal best by four and a half seconds at a meet in Paris.
