Media reports have suggested Chelsea and Atletico agreed a clause in the keeper’s contract that the Madrid team would pay around £5m in compensation if the clubs clashed in European competition.
And Atletico have indicated that they would not be prepared to field him at that price, and would abide by the agreement with the Blues.
But UEFA look certain to overturn Chelsea’s attempts to prohibit the Belgium international taking part in a competitive match between the teams.
A UEFA statement read: “In response to media reports referring to the situation of Club Atlético de Madrid goalkeeper Thibaut Courtois, UEFA would like to reiterate its position.
“The integrity of sporting competition is a fundamental principle for UEFA.
“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.
“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.”