(CNN) -- Tour of Spain runner-up Ezequiel Mosquera and his Xacobeo team-mate David Garcia Da Pena have tested positive for banned substances during the race, the Union Cycliste Internationale (UCI) revealed on Thursday.
The Spanish duo both returned urine samples containing hydroxyethyl starch, a plasma substitute that has the potential to increase blood volume and boost oxygen delivery to the body.
Mosquera and Garcia Da Pena's samples, collected following the September 16 stage of the Tour of Spain, were tested at a World Anti-Doping Agency (WADA) accredited laboratory in Cologne.
Both riders have the right to request that their B samples are tested.
Mosquera went on to finish second behind Italian Vincenzo Nibali in the race earlier this month.
The news came on the same day that Tour de France champion Alberto Contador's positive test for banned drug clenbutrerol was revealed.
The Spaniard has protested his innocence, claiming the substance entered his body through contaminated meat.