Athletes are risking their careers by unknowingly taking banned substances in nutritional supplements, the IOC has warned.

The Olympic governing body has revealed that 15% of the 634 supplements it tested had shown traces of substances such as nandrolone.

Traces of the banned drug were found in a number of the products, despite not being mentioned on the labels.

The IOC, however, insisted sportsmen and women remained responsible for all what they chose to eat and drink.

A statement said: "Under the Olympic movement's rule of strict liability, athletes are responsible for whatever substance is found in their bodies."

Richardson ban

The Netherlands produced the highest level of positive products, 25.8% of the 31 tested, while the United Kingdom was third highest with 18.9%.

The results may account for a number of positive drug tests.

Dutch footballers Edgar Davids and Jaap Stam were both found with the steroid nandrolone in their system while British sprinter Linford Christie also failed a drugs test for the banned substance.

The controversy over nutritional supplements has already caused a series of highly contentious cases.

British 400m runner Mark Richardson was suspended for two years from competition but was reinstated after insisting he had taken contaminated products.