Croatian cyclist Matija Kvasina has been provisionally suspended after two positive tests at the Fleche du Sud.
The 35-year-old won the stage race, part of the International Cycling Union (UCI) Europe Tour.
He tested positive for the banned drug molidustat in tests on May 26 and 28.
The experimental substance stimulates the production of erythropoeitin (EPO) and the production of red blood cells.
Kvasina had been competing at the Tour of Austria for the Felbermayr-Simplon Wels team when informed of the failed tests.
"Yesterday evening Andreas Grossek, the team's sports director, was informed about the positive result of the A-doping test by Matija Kvasina,” a Felbermayr-Simplon Wels statement read.
"The shareholders as well as Andreas Grossek are speechless and respond to this incident with zero tolerance.
“Matija Kavasina was suspended and was sent home immediately.
"The doping sample was taken on a tour at the end of May, and only now published."
Kvasina, winner of Croatia's national time trial title on seven occasions, can request his B-samples to be tested.
He won the Fleche du Sud ahead of Ukraine’s Mark Padun, while Austria’s Riccardo Zoidl finished in third place.
Kvasina has represented Croatia twice in the Olympic Games.
He finished 56th in the road race and 38th in the road time trial at Beijing 2008.
Kvasina also competed at Rio 2016 but failed to finish in the road race.
"For all professional cyclists, doping checks are carried out at regular intervals without notice," said Felbermayr-Simplon Wels in their statement.
"In addition, the team Felbermayr Simplon Wels voluntarily participates in the anti-doping programme 'ADAMS'.
"All the more we are shocked to find this fraud in their own ranks.
"We would like to express our special thanks to the shareholders and team leaders to all the sponsors, who are also clearly behind our team."