The head of Belgium's World Anti-Doping Agency accredited laboratory has dismissed triple Tour de France champion's Alberto Contador's call for a threshold limit for clenbuterol positives.

Contador has been provisonally suspended by the International Cycling Union (UCI) after testing positive for a small quantity of clenbuterol during last July's tour.

He has denied deliberately taking the anabolic agent, which can be used to strip fat and enhance muscle size, and attributed the positive to eating contaminated meat.

In an interview with Reuters Peter Van Eenoo, the head of the Ghent laboratory, said that unlike, for example, the stimulant cocaine clenbuterol broke down very slowly in the body and the effect was long term.

"If I took a lot and you tested me an hour later, I might still have a lot in my system, but if you tested me in four or five days, it might be there in small quantities and you wouldn't know what effect it had had," he said.

"Cocaine is very different, because while it has an enormous immediate effect if you tested me in two days you probably wouldn't detect it.

"That's why cocaine is only banned in competition, and why we don't even report it if it's only there in small quantities. Clenbuterol, though, is always prohibited."

Van Eenoo agreed it was possible for positive clenbuterol tests to come from other sources although athletes were ultimately liable for substances in their bodies.

"Contaminated products or foodstuffs are possible causes, too," he said.