Concern is growing at the Pan Am Games in Mexico over meat that may have been contaminated with steroids, according to reports from Guadalajara.

Clenbuterol, the substance at the centre of a scare in China after more than two dozen athletes tested positive after eating meat there earlier this year, is in focus once more.

Tests conducted in a laboratory in Germany showed that the majority of the players who competed at the FIFA Under-17 World Cup in Mexico earlier this year tested positive for clenbuterol, with 19 out of 24 teams affected. Universal cheating is not suspected. No fewer than 109 out of 208 urine samples had traces of the banned substance in them. Under the circumstances, WADA decided not to escalate any of the cases to doping hearings.

Now in Guadalajara, Games director Carlos Andrade Garin is to be found offering reassurance to athletes as they tuck into their grub at the Athletes' Village.

"We are obviously aware of the problem and of what has happened before [regarding the positive tests from the FIFA Under-17 World Cup] but we are doing our best to control the issue here," he told reporters. "The meat at the Athletes' Village is 100 per cent reliable and we have urged the teams to stick to that. We know where it has come from and we have no doubts. The meat has been analysed and is being watched by police to avoid any chance of contamination so we do not expect problems if the teams follow our advice."

WADA issued a warning for athletes travelling to Mexico "exercise extreme caution" on food, while WADA legal director Olivier Niggli noted the problem at the heart of the meat crisis when he said that it "would be perfect excuse for those who try to use doping".

The problem in Mexico stems from rogue Mexican farmers who illegally inject clenbuterol into their cattle because the drug reduces fat and increases meat. A Mexican offensive is underway to shut down slaughterhouses and arrest farmers who have broken the law.