"He accepted his wrongdoing and did not ask for a B sample," said Eric Boyer, manager of Moreni's Cofidis team.
Athletes who fail a doping test are entitled to ask for a follow-up "B" sample test to confirm — and in rare cases refute — the results of the initial "A" sample.
Police were seen leading Moreni away from the Cofidis team bus. It was unclear where they were taking him. France has tough laws against trafficking in doping products.
Moreni tested positive for testosterone after stage 11 of the Tour last Thursday, said Didier Simon, of cycling's world governing body, the UCI. He said it was for Cofidis to decide whether to pull its other riders from the Tour in the wake of Moreni's failed test.
Danish rider Michael Rasmussen — who has been surrounded by doping controversy himself — won Wednesday's stage and extended his overall lead. He looks increasingly likely to win the race when it finishes Sunday in Paris.
News of Moreni's test came a day after star rider Alexandre Vinokourov and his entire Astana team were sent home after he tested positive for a banned blood transfusion.
Moreni was in 54th place overall at the end of Wednesday's stage, 1 hour, 56 minutes and 11 seconds behind Rasmussen.
The test analysis for Moreni was conducted by the Chatenay-Malabry lab on the outskirts of Paris. Traces of testosterone were found in the urine sample, L'Equipe said. The test showed that the testosterone was administered and that the hormone was not naturally occurring.
Moreni's failed doping test was the latest blow to a race already reeling from doping revelations.
Rasmussen, booed by fans at the start Wednesday, extended his lead in the three-week event, which ends in Paris on Sunday.
The Danish cyclist crossed the finish line alone after the 135.8-mile ride from Orthez to Gourette-Col d'Aubisque, the toughest ride in the Pyrenees this year.
American Levi Leipheimer finished 26 seconds behind, and Discovery Channel teammate Alberto Contador of Spain was third, 35 seconds back. Both lost time against Rasmussen, who broke away from the three-man group in the last half-mile, finishing in 6 hours, 23 minutes, 21 seconds.
"I am one step closer," said Rasmussen, who also won Stage 8 in Tignes.
That's worrisome to some fans and dozens of riders who staged a silent protest against the doping scandals in their sport — delaying the start by 13 minutes.
Previously, Tour rider Patrick Sinkewitz had tested positive for elevated levels of testosterone. Sinkewitz has denied doping and asked for his B sample to be tested, with the results expected to be known by July 29.
Sinkewitz tested positive in training on June 8 — a month before the Tour started — but he competed in the race until he crashed into a spectator during the eighth stage on July 15.