Two 3000m steeplechasers, Nordine Gezzar of France and Spain's Angel Mullera, were both axed from the Olympic Games on Friday on suspicion of doping.
French runner Nordine Gezzar could face a life ban after testing positive for the banned blood booster EPO (erythropoietin).
Gezzar has requested a "B" sample to be analysed, with the results of that test due to be known next week.
Having already served a two-year drug ban when he was suspended in 2006, Gezzar could be banned permanently should the second control also come back positive.
Michel Marle, French athletics federation vice-president, told AFP on Friday: "Gezzar tested positive for EPO and has been provisionally suspended."
"The athlete has already requested the analysis of the 'B' sample. That will be carried out on Sunday and we'll probably know the results 48 hours later."
Gezzar, fourth at the European Championships in Helsinki, was caught out when he was tested along with all the other Games-bound competitors by the French Anti-Doping Agency.
The 32-year-old long distance runner was handed a two-year suspension in 2006 when testing positive for the banned anabolic steroid nandrolone and the diuretic finasteride, which is used to treat hair loss but can also mask steroid use.
Later news emerged from Madrid concerning Mullera with the Spanish Athletics Federation (RFEA) announcing that he would also not be competing at London 2012.
The RFEA took the decision to remove Mullera from their squad after reports in the local media also connecting him with the use of EPO.
He has been replaced by Sebastian Martos.
"This decision is independant of the eventual opening of a disciplinary process accompanied by a preventive ban," the RFEA said.