Madrid has admitted that it wants to buy the Portugal winger, whose 42 goals for the Red Devils this season helped United win the Premier League and Champions League trophies.

Although Ronaldo is only 13 months into a five-year deal, Madrid is reported as saying it is willing to pay a world-record transfer of about 60 million pounds (US$118.8 million; 75.6 million) for the winger with the dazzling footwork and a powerful shot.

Tired of seeing headlines on a daily basis about Ronaldo's future, United issued a statement on Tuesday threatening to take action against the Spanish club.

"Manchester United has watched with growing irritation the comments attributed to Real Madrid over their alleged desire to sign Cristiano Ronaldo," the club said on its Web site. "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."

United said Madrid's attempts to buy Ronaldo are also a distraction for the Portuguese national squad as it prepares for next month's European Championship.

"No one should be in any doubt that Manchester United will do everything in its power to keep its best players," the club said.