Contaminated meat in Mexico led to traces of the banned drug clenbuterol being found in urine samples given by more than 100 players involved in the FIFA Under-17 World Cup in June, the world governing body's medical chief has revealed.
Of the 24 squads involved, 19 - possibly including England's youth team - had several players showing the presence of clenbuterol but at concentrations lower than the prohibited level.
Positive tests for five players from the senior Mexico squad had alerted FIFA to a possible issue, and when four more positive tests emerged from the youth tournament the governing body decided to reanalyse all the 208 urine samples taken.
A laboratory in Cologne discovered the presence of the steroid in 109 of those samples - 52.4% - but most in concentrations lower than the banned level so they had not been reported. Clenbuterol is banned in farming in most countries but is used to speed up growth and increase muscle mass in cattle.
FIFA's chief medical officer Jiri Dvorak said players from 19 of the 24 squads, in roughly equal numbers, had shown traces of the drug. Dvorak would not say whether England's under-17 squad was one of those involved but insisted no harm had been done to the players.
He said: ''FIFA was very alarmed and it was highly surprising to see something like this - I had not seen anything like it in my 20 years in this post. My first question was, 'could any harm have been done to the players?' and I was assured by the different medical specialists the answer is no.''
Dvorak said that as early as April the German anti-doping organisation had issued a warning that athletes should exercise caution in Mexico and China because of adverse analytical findings due to meat contamination.
FIFA ordered meat samples to be collected from team hotels and 30% of these showed the presence of clenbuterol.
The Mexican government have made a number of arrests and closed down several slaughterhouses in recent weeks after being alerted to the issue, according to Mikel Arriola, an official from Mexico's health ministry. Mexico's victorious under-17 team did not have a single adverse finding; after the positive tests for the senior players they were only allowed to eat fish and vegetables.