Tour de France champion Alberto Contador has been provisionally suspended by the International Cycling Union (UCI) after testing positive for a banned substance.

Contador tested positive for clenbuterol on July 21 in Pau, during the second rest day of the Tour de France, but has blamed it on food contamination.

Following the rest day the Spaniard set up his third Tour de France victory with a thrilling battle with eventual runner-up Andy Schleck on the famous Col du Tourmalet.

Contador won the Tour by just 39 seconds from Schleck.

The UCI confirmed that only a "very small concentration" of the drug had been found and that the case warranted "further scientific investigation" because the Cologne laboratory that detected the substance is known to be able to detect the tiniest traces of drugs.

"The rider, who had already put an end to his cycling season before the result was known, was nevertheless formally and provisionally suspended as is prescribed by the World Anti-Doping Code (WADA)," a statement said.

"The concentration found by the laboratory was estimated at 50 picograms which is 400 times less than what the anti-doping laboratories accredited by WADA must be able to detect."

The Spaniard is set to admit the positive test at a press conference from his home town Pinto at 8.00pm today AEST.

A message from Contador's personal press officer says the cyclist was a victim of his food being spiked.

"Alberto Contador is affected by a doping control at the last Tour de France on July 21, where it was found the substance clenbuterol," the message reads.

"From the time of the first communication from the UCI, August 24, Alberto Contador alleged food contamination as the only possible explanation of what happened and has been turned over to the cyclist authorities since then in the confidence that this very serious problem could be clarified, which now is public.

"The experts consulted so far have agreed also that this is a food contamination case, especially considering the number of tests passed by Alberto Contador during the Tour de France, making it possible to define precisely both the time the emergence of the substance as the tiny amount detected, ruling out any other source or intentionality.

"Alberto Contador will offer ... a press conference in order to give his version of what happened to the public."

Clenbuterol is often used to treat breathing disorders such as asthma, and increases the user's heart rate, temperature, perspiration and blood pressure.

The news casts a massive shadow over the cycling world championships which are currently being held in Geelong, as well as a sport that has been plagued by a string of drug cheats.

The news also provides some reasons as to why Contador snubbed the event in which Australia's Cadel Evans is the reigning champion in the road race.

American Floyd Landis is the only rider to be stripped of a Tour de France victory after testing positive to testosterone after winning the 2006 edition.

Contador was part of the Astana team during this year's Tour de France but has since secured a move to rivals Saxo Bank.