Research released on the eve of the Beijing Olympics has revealed as many as one in 10 dietary supplements could contain prohibited substances not listed on the label.

The study examined 152 supplements sold in the UK and found 10.5% of them contained a steroid or stimulant banned by the World Doping Agency.

The research was conducted by specialist drug-screening laboratory HFL Sport Science and funded by energy drinks brand Lucozade Sport.

Responding to the research, Andy Parkinson, acting director of drug-free sport at government agency UK Sport said: "We know great strides have been made by many manufacturers in ensuring the supplements they produce are not contaminated. However, the research highlights the fact this isn't the case across the board and emphasises how important it is for athletes to consider their options with regards to the use of supplements."

"Our message to athletes is clear — ignorance is not an excuse when it comes to supplements and strict liability still applies. If you test positive as a result of using a contaminated supplement you will face a sanction under the World Anti-Doping Code and could be looking at up to a two-year ban. We therefore encourage all athletes to minimise the risk by taking the time to make sensible, informed decisions about supplements."

In the wake of publication of the research, it emerged US swimmer Jessica Hardy had tested positive for clenbuterol, a prohibited anabolic agent, in July. She withdrew from the US Olympic team, but claimed she had not knowingly taken the banned substance and vowed to clear her name. Media reports have suggested she might blame a tainted dietary supplement or sabotage for its presence. For more on that story, click here.