United condemned Real's behaviour as "totally unacceptable" just hours after Real coach Bernd Schuster described Ronaldo as: "The big priority for the club."
The Spanish giants have publicly declared their interest in the winger, saying they would be prepared to pay £100 million for him.
A statement from Old Trafford described the club's "growing irritation" at the Liga champions' behaviour, adding:
"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."
United believe repeated statements of interest in Ronaldo constitute a violation of 'tapping-up' laws, which require a buying club to agree a transfer fee before it can contact a player on another team.
It is the latest twist in an increasingly acrimonious saga, with Madrid newspaper Marca - which has close ties to Real - printing daily articles claiming Ronaldo will sign.
United chief executive David Gill met with Real president Ramon Calderon last week to warn him off the 23-year-old, who scored 42 goals this season.
The Premier League champions' manager Sir Alex Ferguson said Real's pursuit of his prize asset showed they are "moral-less", and even compared them to former Spanish dictator General Franco.
Calderon said he was perplexed at United's negative reaction, alleging that his club have behaved "exquisitely".
Ronaldo himself has done little to quell speculation. He has said he is happy at Old Trafford but has pointedly refused to pledge his future to the club, and recently said it is his "dream" to play in Spain.
The Portuguese forward's extraordinary performances this season have earned him numerous inidividual accolades and played a major part in United's double trophy success.
He is the front-runner for the World Footballer of the Year and on present form the only player who can deny him is Lionel Messi of Real's arch-rivals Barcelona.
დაიგრალის
This post has been edited by Rozer on 28 May 2008, 13:30 Truth is, everybody is going to hurt you; you just gotta find the ones worth suffering for." -Bob Marley
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