China's ban on staple meats for its Olympic athletes has raised concern over its effect on their performance as the London Games approach in less than a month.

The Sports Ministry instituted a ban on most beef, lamb and pork in January with the aim of preventing athletes from consuming meat tainted with clenbuterol, which is classified as a performance-enhancing drug by the International Olympic Committee and is grounds for disqualification if detected.

Clenbuterol is a prohibited substance and there is no threshold under which this substance is not prohibited, read a statement from the World Anti-Doping Agency, which administers drug-testing for the IOC.

Some Chinese coaches and officials have expressed concern that the ban is depriving athletes of the nutrition they need to perform. The coach of the women's volleyball team blamed the ban for a set of losses in a recent international tournament after the team had been on a vegetarian diet for three weeks.

They have showed significant decline in their strength and fitness, Yu Juemin told Beijing News after the team sustained four losses in the World Grand Prix in Ningbo, China. We dared not eat pork when we came out of our training camp for the tournament because we were afraid of clenbuterol.

Athletes are still allowed to eat meat screened by their training centers, and have also added more fish and protein powder supplements to their diets in response.

WADA issued a warning last November regarding clenbuterol-tainted meat, particularly in China and Mexico.

It has been shown that Mexico and China have a serious problem with meat contaminated with the prohibited substance clenbuterol, and WADA's message to athletes competing in these countries remains the same: Eat only in restaurants and cafeterias that have been approved by your federation and/or event organizer, the statement read.

Chinese athletes have been advised to order chicken or fish when eating out as a significant portion of beef, lamb and pork contains clenbuterol, which is used to produce leaner meat in livestock.

A number of Olympic athletes have previously been disqualified for having clenbuterol in their systems, though in some cases it was acknowledged that it was accidental.

It is possible that under certain circumstances the presence of a low level of clenbuterol in an athlete sample can be the result of food contamination, read a statement from WADA. However, each case is different and all elements need to be taken into account.