The drug GW501516 was abandoned by manufacturers GlaxoSmithKline in 2006 after clinical trials involving lab rats showed that it caused tumours in a number of organs, including the liver, bladder, stomach, skin, thyroid, tongue, testes, ovaries and womb.

WADA are now concerned that athletes are purchasing the fat burner on the black market. 

"It has come to WADA's attention that the black market substance GW501516 is being sold to and used by some athletes," WADA said in a statement. 

"The side effect of this chemical compound is so serious that WADA is taking the rare step of warning 'cheats' to ensure that there is complete awareness of the possible health risks to athletes who succumb to the temptation of using GW501516 for performance enhancement.

"GW501516 was a developmental drug that was withdrawn from research by the pharmaceutical company and terminated when serious toxicities were discovered in pre-clinical studies.

"Clinical approval has not, and will not be given for this substance.

"However, GW501516 has been available for some months on the black market, through the Internet and elsewhere.

"Anti-Doping authorities have already seen its use by athletes, as there are a number of positive cases."

WADA refused to reveal which athletes had tested positive for the drug but details are expected to emerge shortly.

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