Despite the latest heavy blow to a sport reeling from doping scandals, Tour de France organizers said the race would go on.
The Kazakh rider, a one-time favorite to win cycling's premier event, was tested after his victory in the 13th stage time trial on Saturday.
"Vino has tested positive having to do with a blood transfusion and the team is leaving the Tour," team spokeswoman Corinne Druey said, using the rider's nickname.
Vinokourov, a pre-race favorite, won two stages this year - the time trial in Albi and Monday's 15th stage. He is 23rd in the overall race standings. The Tour finishes in Paris on Sunday.
Once seen as a favorite to win the Tour, Vinokourov dropped out of contention for good Sunday after losing 28 minutes, 50 seconds to race leader Michael Rasmussen.
The French sports daily L'Equipe, which first reported the positive test on its Web site Tuesday, said the analysis was conducted by the Chatenay-Malabry lab on the outskirts of Paris. It said two distinctive types of red blood cells were found in the A sample and showed that Vinokourov received a blood transfusion from a compatible donor shortly before the time trial.
A senior French anti-doping official confirmed to The Associated Press that there was a positive test for a blood transfusion taken from a rider at the Tour on Saturday, but said he didn't know the name of the cyclist involved. He said the test found two different types of blood, one from the rider, one from a donor.
The official spoke on condition of anonymity because no official announcement had been made.
The president of cycling's world governing body, the UCI, said he could not comment as long as the result of the backup B-sample had not been confirmed.
"We have a process in place and we have to see this process through," Pat McQuaid told The Associated Press in a telephone interview
Vinokourov has been a crowd favorite along the course route this year. He was injured in a crash in the fifth stage, requiring stitches in both knees.
"With a guy of his stature and class, in cycling's current situation, we might as well pack our bags and go home," said British rider David Millar, who came back from a two-year doping ban in the Tour last year.