Spanish rider Alberto Contador returned an "adverse analytical finding" for clenbuterol following an analysis of a urine sample taken during an in-competition test on the second rest day of the Tour de France, the International Cycling Union said on Thursday.
This result was reported by the World Anti-Doping-Association accredited laboratory in Cologne to UCI and WADA simultaneously.
The concentration was "400 time(s) less than what the antidoping laboratories accredited by WADA must be able to detect," the UCI said in a statement.
"In view of this very small concentration and in consultation with WADA, the UCI immediately had the proper results management proceedings conducted including the analysis of B sample that confirmed the first result."
Contador, who won his third Tour de France this year, had been formally and provisionally suspended as is prescribed by the World Anti-Doping Code, the UCI said.
The case would require "further scientific investigation before any conclusion could be drawn."
The investigation could take some time, it said.
The UCI had no further comment.