The European Commission has warned Fifa, the game's world governing body, not to seek "gentleman's agreements" to limit foreign players at clubs, which risk costly court cases like the Bosman ruling.
"EU laws are superior to any such gentleman's agreements," an official at the EU executive familiar with the matter said. "I would advise Fifa to think carefully and maybe to think twice about such a move."
The official was responding to reports last week in which Fifa officials said president Sepp Blatter will try to bypass EU labour laws by reaching a deal with national associations at Fifa's annual congress next month to limit teams to just five foreign nationals.
"If say Chelsea were to be blocked from buying Ronaldinho because of such a deal between Fifa and the English FA, the player or Chelsea would be well within their rights to challenge such an agreement under EU law," another official said. "Footballers are treated as workers and must be allowed access to work in all member states by national authorities."
Football's European governing body, Uefa, has also warned Blatter over his proposals, which conflict with EU laws on the free movement of workers, pointing out that any move to outflank Brussels could potentially lead to widespread legal actions.
Uefa wants a deal with Brussels on its home-grown player rule which sets a quota of locally trained players at clubs but without any discrimination on nationality and to avoid a repeat of a 1995 ruling by the European Court of Justice – Europe's top court – which was damaging to the sport.
Better known as the "Bosman ruling", named after Belgian player Jean-Marc Bosman, the ECJ gave all sports professionals within the 27-member bloc the freedom to change clubs, much to the anger of football's authorities.
"We have to see how it plays out, but yes, this could be challenged in the national courts and if necessary end up at the ECJ which, like Bosman, could prove very costly," the second Commission official said.
To change Fifa's rules, Blatter needs 75 per cent support at the congress in Sydney of Fifa's 208 member associations.