Tour de France champion and Team Saxo Bank-SunGard rider Alberto Contador on Friday called for anti-doping regulations to be revised after his positive test for a banned anabolic agent, saying the whole system was in question.

The 27-year-old Spaniard said "the system is in doubt and should be changed."

"There has to be a limit set for substances like clenbuterol so that quantities as tiny as those found in my body due to contaminated food do not count as a positive."

The International Cycling Union (UCI) said on Thursday Contador had been provisionally suspended after a positive test for a very small concentration of the banned anabolic agent.

In a damaging day for cycling's already battered image, the Tour of Spain runner up Ezequiel Mosquera and his Xacobeo team mate David Garcia Da Pena were also suspended for suspected doping.

Contador categorically denied rumours of a possible blood transfusion during the Tour de France.

"If they want to test every sample I've given in the Tour, as many different laboratories as they want, or if they want to freeze it for three or five years until other future tests are scientifically validated and then check it, they can do it," he said. "I have nothing to hide."

Responding to comments by WADA (World Anti-Doping Agency) director-general David Howman that there is no threshold for a positive dope test for clenbuterol, Contador said:

"There should be ... the norms have to evolve, just as they have done for other substances like caffeine, where they changed the regulations because they realized they weren't right."

"In the case of clenbuterol, positives should be positives because of the quantity found, with a specific limit, not because of the substance itself."

Contador repeated that the presence of clenbuterol in his system, in an amount 40 times less than the minimum required for authorized anti-doping labs to be able to detect, was due to contaminated meat containing minute traces of the substance.

"It just can't be that positives for contaminated food stuff like mine are placed in the same category as a standard positive for doping," he said.

"What I want is for all of these doubts and suspicions, even the slightest ones, to be cleared up completely and permanently," Contador added.

Contador said he was suffering mentally because of the stress of the allegations.

"I feel like I'm at rock bottom. I feel really let down. I'm fighting against these accusations 24 hours of each day."

"Right now I'm in a place I never imagined I would be, and it's not good," he concluded.