Kolobnev now faces the prospect of being sacked by Katusha
Russian cyclist Alexandr Kolobnev has been suspended by the Katusha team after his B sample tested positive for banned diuretic hydrochlorothiazide after stage four of the Tour de France.
The 30-year-old, who has twice won world road race silver and an Olympic bronze in 2008, is the first rider to fail a doping test in the 2011 race.
"Since Kolobnev's B sample came back positive, Team Katusha confirms the suspension of the rider," said Katusha.
"Kolobnev formally suspended himself."
A decision on whether Kolobnev will be fired will be made after the Russian federation reviews the case, the team added.
Kolobnev could potentially face a two-year ban from the sport.
Hydrochlorothiazide is listed as a banned substance because it can also be used to mask other drugs.
Kolobnev was 69th in the overall standings after nine stages, 22 minutes and 15 seconds behind leader Thomas Voeckler.
He won a bronze medal in the road race at the 2008 Olympics after being promoted from fourth after second-placed finisher Davide Rebellin failed a drugs test.
Kolobnev won World Championship silver medals behind Paolo Bettini in 2007 and Cadel Evans in 2009.