"We have watched with growing irritation the comments attributed to Real Madrid over their alleged desire to sign Cristiano Ronaldo," said the statement on the club's official website.
"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."