The Premier League and European champions issued a statement this evening describing their "growing irritation" at continuing comments seemingly emerging from the Spanish club concerning their interest in the Portugal attacker.
The statement, released on their website, manutd.com, said: "Manchester United has watched with growing irritation the comments attributed to Real Madrid over their alleged desire to sign Cristiano Ronaldo.
"The facts are: the player is on a long-term contract and his registration is held by Manchester United; The player is not for sale.
"The club will have no alternative but to report Real Madrid to the world governing body, FIFA, if it continues to behave in this totally unacceptable fashion. These public attempts to unsettle the player are completely against the regulations and the club will not tolerate them any longer.
"In addition, the club is certain that they are a distraction for the Portuguese national squad as they prepare for the European Championships. No one should be in any doubt that Manchester United will do everything in its power to keep its best players."