Plans by world football's governing body FIFA to introduce nationality quotas on the football pitch were condemned as illegal by the European Commission today.
FIFA is considering a so-called "six and five" rule ensuring that at least six players in a squad are from the club's own country.
The plan is due to be put to a FIFA congress in Sydney on Friday.
However, today the Commission stepped in to warn that such a move would amount to discrimination - outlawed by EU rules.
"We are giving red card to the six and five rule," announced Vladimir Spidla, Europe's Commissioner for Equal Opportunities.
FIFA's controversial proposal is different from a "home grown players" rule proposed by UEFA, the sport's European governing body.
UEFA defines "home grown players" as team members who, regardless of age or nationality, have been trained by their club or by another club in the National Association for at least three years between the age of 15 and 21.
The Commission has approved the UEFA arrangement because, says Brussels, it contains no player conditions based on nationality.
"Compared with the intentions announced by FIFA to impose the so-called 'six and five rule' which is directly discriminatory and therefore incompatible with the EU law, the 'home grown players' rule proposed buy UEFA seems to me to be proportionate and to comply with the principle of free movement of workers".
Today's Commission announcement follows an independent study on the 'home grown player rule' under which clubs taking part in the Champions League and the UEFA Cup would have to have a minimum number of 'home grown players' in their squads.
A Commission statement said the potential risk of the rule leading to indirect discrimination on the basis of nationality could not be discounted. The underlying objectives of the UEFA scheme - to promote training for young football players - were legitimate.
EU Commissioner for Education, Training and Youth, Jan Figel, commented: "Measures which require the top European clubs to preserve quality training structures seem to me to be necessary. The UEFA rule thus avoid the risk of professional football clubs abandoning training structures."