Cycling's governing body said Schleck tested positive for a banned diuretic. His team said he was cooperating with French authorities at a police station.
The 32-year-old RadioShack Nissan Trek leader finished third in the Tour last year. He was 12th overall -- 9 minutes, 45 seconds behind leader Bradley Wiggins -- on Tuesday's rest day. His brother Andy was awarded the 2010 Tour victory after Alberto Contador was stripped of the title because of his positive test for clenbuterol, but is sitting out this year's race with a spinal injury.
UCI, the governing body, said the diuretic Xipamide turned up in a drug test conducted by the French lab in Chatenay-Malabry on a sample from Schleck taken July 14.
The RadioShack team said it decided to withdraw Schleck from the race, adding the diuretic is not in any medicine used by the squad and why it was detected in Schleck is "unclear to the team." Therefore, the team said in its statement, it is "not able to explain the adverse findings at this point."
Team spokesman Philippe Maertens said Schleck had gone to the Pau police station of his own accord to cooperate with authorities. Maertens said the rider knew police would likely be coming for him.
Schleck has the right to request a test of his B sample. The timeline allows four days for Schleck to have the backup sample analyzed, UCI said.
RadioShack said it will continue to compete in the race. But it was more bad news for the squad that was built on the remains of former teams of Lance Armstrong, who helped land the top-line sponsorship of the American retail chain for the team.
The team manager, Johan Bruyneel, has been targeted in the same U.S. anti-doping case targeting the seven-time Tour champion. Bruyneel skipped the Tour to avoid being a distraction to the race and RadioShack riders.
The case is also likely to cast new doubt on cycling's ability to root out drugs cheats despite vigorous controls put in place by the UCI and its allies in the anti-doping fight. It is the second doping case to emerge at the Tour this year. Cofidis rider Remy Di Gregorio of France was arrested on the first rest day on July 10 as part of a Marseille doping inquiry.
The diuretic is classified as a specified substance and does not require a provisional suspension. The World Anti-Doping Agency defines specified substances as those that are "more susceptible to a credible, nondoping explanation." Bans for such substances are often shorter, and athletes have a better chance of proving they did not intend to consume it or enhance their performance.
Contacted by phone by The Associated Press in Mondorf-les-Bains, Luxembourg, Frank's 36-year-old brother Steve said he had tried to reach the RadioShack rider by phone but was not successful.
"We're a little bit shaken up," Steve Schleck said.