Fifa's plan to introduce a quota on foreign players has been struck a major blow by the European Commission.
Blatter's player quota plan may not see the light of day
Sepp Blatter, head of football's ruling body, was hoping to enforce his 6+5 rule that would stop clubs from playing more than five non-nationals in an XI.
"The implementation by Fifa of a 6+5 rule in the European Union would violate EU law," said an EC spokesman.
"The Commission is not considering any change to allow Fifa to push forward this idea. Fifa is aware of this fact."
A spokesman for football's governing body said they were not going to comment on the statements issued by the EC, who are the executive branch of the EU.
Despite the stance, Blatter is set to submit his idea to the Fifa Congress when it meets in Sydney on 29 and 30 May.
"Over the years and decades, by signing more and more foreign players, clubs have gradually lost their identity, first locally and regionally," said the Fifa president when explaining his proposal.
A Fifa statement said that Blatter "will not tackle supranational organisations such as the EU but rather convince them of Fifa's approach".
Blatter is holding out hope that the mention of the specificity of sport in a new European Treaty will give concession to his idea.
But the EC said that would not happen and that his plan would still be considered "illegal".
"It would appear that the wording of the new provision on sport in the Treaty is causing Mr Blatter and colleagues to conclude that the quota idea may yet see implementation because the new Treaty also mentions the 'specificity of sport'," continued the EC statement.
"It is extremely unlikely that the new Treaty provision on sport would lead to a new legal situation.
There now needs to be a very, very careful analysis of what the legal requirements are or are not about the employment of players
FA chairman Lord Triesman
"Hence, a system of quotas based on nationality, as proposed by Blatter, is illegal under the current acquis and will remain illegal under the new Treaty."
Blatter's concept has already been met with concerns from Celtic chief executive Peter Lawwell, who said the club will "absolutely" oppose the plan.
"Within football at the moment there are already enough restrictions being imposed on clubs without further ones being applied," he added.
The Premier League also reiterated its opposition to the proposal during Tuesday's Culture, Media and Sport Committee meeting in London.
But Football Association chairman Lord Triesman hinted that the introduction of quotas should be given consideration.
"There now needs to be a very, very careful analysis of what the legal requirements are or are not about the employment of players but also what may be the regulations about how many start in games - which may not be the same," he stated.