The governing body issued a statement on Friday amid reports Atletico Madrid goalkeeper Thibaut Courtois could be prevented from playing should the Liga leaders draw his parent club Chelsea in the UEFA Champions League.
The Belgium international is in his third consecutive season-long loan deal at Atletico from Chelsea, and with both teams progressing to the last four of the competition, there is a real possibility they will face each other.
Atletico president Enrique Cerezo is quoted as saying that a clause in Courtois' loan contract stipulates that "a big quantity of money" must be paid to Chelsea to allow the keeper to face his parent club, and UEFA have now underlined their stance on the matter.
European football's governing body appeared to prohibit any such clause and stated, if a stipulation of that nature was enforced, sanctions would follow.
"Both the UEFA Champions League and the UEFA Disciplinary Regulations contain clear provisions which strictly forbid any club to exert, or attempt to exert, any influence whatsoever over the players that another club may (or may not) field in a match," the statement read.
"It follows that any provision in a private contract between clubs which might function in such a way as to influence who a club fields in a match is null, void and unenforceable so far as UEFA is concerned.
"Furthermore, any attempt to enforce such a provision would be a clear violation of both the UEFA Champions League and the UEFA Disciplinary Regulations and would therefore be sanctioned accordingly."
