Tour de France shaken by another doping scandal
The 32-year-old from Luxembourg was immediately withdrawn from the race by his RadioShack team. Schleck was in 12th place and finished third overall last year.
It's the second doping scandal to hit this year's Tour. Cofidis rider Remy Di Gregorio of France was arrested on July 10 as part of a Marseille drugs probe.
With just five days to go until the finish, the race heads into the Pyrenees today, and Bradley Wiggins will be looking to protect his two-minute lead at the top of the standings.
His Sky team-mate Chris Froome is second while Italy's Vincenzo Nibali is third a further 23 seconds behind.
Wiggins was as cool as ever during Tuesday's rest day even though he is likely to be attacked by his rivals soon.
“I don't really fear anything,” he told reporters. “It's really just a case of going out and doing the performance. … Ultimately it's just a bike race.”