The former world number one, who now lives in Hong Kong, stressed again last week that the low trace of clenbuterol found in her body - 50-100 pg/ml (picograms per millilitre) - came from unwittingly consuming contaminated pork.

According to Dr Lam Hon-wah, of City University, the amount is inconsistent with what could be found in an athlete who regularly ingests clenbuterol for performance-enhancing purposes. However, clenbuterol does not occur naturally, and is banned at any level by the World Anti-Doping Agency (WADA).

The incident sent a stern warning to Hong Kong athletes, as many of them spend a lot of time at training camps and competitions on the mainland, where the consumption of pork or meat-related food products is unavoidable.

Dr Julian Chang, a sports medicine expert and medical adviser to the Hong Kong Olympic Committee, urged athletes to take extra caution with what they eat. 'WADA states very clearly that athletes have to take the responsibility for what they ingested,' Chang said. 'That means even if in the case of inadvertent doping, they still face sanctions.'

The danger is widespread 'because you don't know whether you have consumed meat or pork that is contaminated', Chang said. 'Clenbuterol is prohibited for use in farm stock but there is still a lot of illegal usage, especially in some backward areas. There is always the possibility of eating tainted pork or other meat that will cause clenbuterol to be found in the body.'

The Hong Kong Anti-Doping Committee (HKADC) advises athletes to avoid eating cattle and pig offal like lung, liver and kidney, as clenbuterol tends to accumulate there. And it urges athletes to buy meat and food from reputable stores or restaurants.

'In an extreme case found in Tianjin, their judo team even raises their own pig farm near the training campus to ensure pork eaten by their athletes is free from clenbuterol,' said Dr Yvonne Yuan Wai-yi, head of the HKADC. 'That may sound funny, but it proves athletes have to be extremely careful about their choice of food.'

The famous German Sport University Cologne issued a warning to athletes in February regarding clenbuterol use in China. An investigation presented at the Cologne Workshop on Doping Analysis analysed urine samples of 28 travellers returning from China to Germany. There were low concentrations of clenbuterol in 22 of them.

The university warned of 'risks of inadvertent doping with the beta2-agonist clenbuterol when travelling to China'.

In September's Tour de Beijing, one of the World Tour series, cyclists were so scared of testing positive for clenbuterol that nearly the entire peloton refused to eat pork or beef during the week-long race.

Chinese authorities hit back at the accusations, saying the Germans were overreacting.

Zhou Jian , deputy director of the China Anti-Doping Agency, argued that China stages 100 to 200 domestic and international competitions a year, and the problem 'would have been exposed long before'.

'We select 1,500 athletes at random for drug testing every year and the results rarely appear positive,' Zhou added. He said food contamination is a global problem and the German university should not 'mislead athletes with simple and separate statistics'.

However, in March, the mainland launched a crackdown on clenbuterol use in meat products. The Ministry of Commerce took measures to block pork containing the illegal additive from entering the market.

Many top mainland athletes have avoided pork because of the scale of the problem.

On April 18, the General Administration of Sports banned the national swimming team from eating at unauthorised restaurants, mainland media reported. The reason given was to protect them from possible contaminations of clenbuterol.

'Many sports training centres now have their own pig farms,' Zhang Yadong, China's former national swimming coach, earlier told local media. 'The sports academy where I work has this long-term contract with a local pig farm and we send people there regularly and check every batch of pork they send to us. You have to be really careful.'

Zhang Lin, China's most successful swimmer, openly condemned the use of clenbuterol on his personal blog: 'The government should take the opportunity [of recent scandals] to weed out these unscrupulous traders ... the Chinese public in the past knew very little about clenbuterol. Now we should all become aware of its damages. It can ruin many top athletes' careers. Now we can't even be certain of what we can eat and what we can't.'

In Henan province, the commerce ministry urged Jiyuan Shuanghui, a subsidiary company of the country's top meat processor Shuanghui Group, to suspend production and launch in-house investigations after pigs were found feeding on food laced with clenbuterol to produce lean meat, which sells for a premium on the mainland.

The ministry also issued an urgent circular requiring local offices to conduct thorough examinations of slaughterhouses and to put strict quality inspection systems into effect.

In late October, Liaoning province - the host of the 2013 National Games - launched a two-year crackdown on the use of clenbuterol in pork and meat-related products in order to create a safe environment for the Games.

After the German anti-doping agency issued its warning, the Hong Kong Sports Institute (HKSI) and its nutrition department stopped providing pig offal to its elite athletes as a precaution.

'Clenbuterol cannot be produced by the human body and once it is found in the system, it must be taken from outside whether it is unintentionally or because of doping purpose. That's why the WADA has taken a non-threshold policy towards this banned substance,' Yuan said.

'However, there are only a few accredited laboratories in the world [less than five] that will detect minute traces of clenbuterol like the one in Barcelona that handled Zhou Mi's sample.'

That may explain the Chinese official Zhou's claim that their tests of 1,500 athletes every year rarely detect clenbuterol; the laboratory in Beijing does not test clenbuterol to such a minute level.

But with more and more cases of low levels of clenbuterol being found in doping tests, particularly in countries such as China and Mexico where clenbuterol is widely used illegally in livestock farming, Yuan said WADA was reviewing the situation.

The high-achieving Hong Kong cycling team, however, gives only scant concern to the perils of tainted pork or meat products, even though its riders train almost year-round on the mainland.

'Food contamination is not only a problem of China. It happens in different parts of the world,' said Fred Chan Chun-hung, general secretary of the Hong Kong Cycling Association. 'There are sufficient doping programmes carried out by relevant bodies such as the HKSI and the International Cycling Union.

'And we don't worry too much about sending our team training regularly in China.'