Former Olympic road race champion Samuel Sanchez has tested positive for a growth hormone-releasing peptide, the UCI has announced.
The 39-year-old Sanchez, who won his gold medal in Beijing in 2008, rides for BMC Racing and has been provisionally suspended after the failed drug test, cycling's world governing body added.
A UCI statement said: "The Union Cycliste Internationale (UCI) announces that Spanish rider Samuel Sanchez Gonzalez was notified of an adverse analytical finding of GHRP-2 in a sample collected in the scope of out-of-competition control on August 9, 2017.
"The doping control was planned and carried out by the Cycling Anti-Doping Foundation, the independent body mandated by the UCI, in charge of defining and implementing the anti-doping strategy in cycling.
"The rider has the right to request and attend the analysis of the B sample.
"In accordance with UCI anti-doping rules, the rider has been provisionally suspended until the adjudication of the affair."
Growth hormones have been shown to reduce fat, increase lean body mass and improve aerobic performance.
Sanchez had been listed to start the Vuelta in Nimes on Saturday but has now been withdrawn and suspended by BMC Racing.
A statement from the Swiss-American team read: "In accordance with BMC Racing Team's zero tolerance policy and UCI regulation, Sanchez has been provisionally suspended with immediate effect.
"All riders and staff are held to the highest ethical standard and BMC Racing Team is extremely disappointed to share this news on the eve of the Vuelta a Espana. "
Loic Vliegen will replace Sanchez at the Vuelta a Espana.