NARBONNE, France  The Tour de France, already diminished by the absence of some of the world’s best cyclists because of doping violations, was thrown into turmoil yet again Thursday when an Italian rider who had won two stages of this year’s race was kicked out of the competition for failing a drug test.

The rider, Riccardo Ricco, is the third cyclist in this year’s race to test positive for the banned blood-boosting drug EPO. Upon the announcement of his failed test, his Saunier Duval-Scott team, which is based in Spain, voluntarily withdrew from the Tour.

Ricco, an outspoken 24-year-old rider who was ninth in the overall Tour standings, is by far the most visible cyclist to be caught doping this year at cycling’s biggest event. He was also in the lead in the competitions for the best mountain climber and for the best rider age 25 or younger. In May he finished second in the Tour of Italy.

The new doping case raises questions about whether antidoping authorities are making progress in their fight. This year is the 10th anniversary of the Festina scandal, which similarly shook the Tour de France when an official of a French cycling team was arrested after being caught with a car full of EPO and related doping products.