Neither the rider's identity nor the date when the test was conducted were released by the sport's governing body.

The positive test, carried out by France's national anti-doping laboratory at Chatenay-Malabry, will be the first from the race if confirmed.

"The adverse analytical finding received this morning relates to the first analysis," the UCI said.

"It will have to be confirmed either by a counter-analysis required by the rider, or by the fact that the rider renounces to that counter analysis.

If the B sample turns out to be positive, I'll be angry UCI president Pat McQuaid

"The World Anti-doping Code and the Anti-doping Rules of the UCI do not allow to make the name of the concerned rider public, as well as other information that may allow identification."

The UCI said the rider, his team, the national federation and the national and world anti-doping authorities had been informed.

UCI president Pat McQuaid said he would be dismayed if the test was confirmed but vowed no let-up in the war on drug cheats.

"From my point of view it's intolerable and we're determined to root out this problem and deal with it," McQuaid said.

"At the moment I'm concerned over the A sample result. If the B sample turns out to be positive, I'll be angry."

This year's Tour was rocked by a drugs scandal on the eve of the race which saw 13 riders, including pre-race favourites Jan Ullrich and Ivan Basso, withdrawn from the race.

They were on a list of 56 riders implicated after a Spanish blood-doping investigation, named "Operation Puerta".