Less than a week after Sir Alex Ferguson expressed his hostility to Real's public pursuit of Ronaldo, tempers within Old Trafford boiled over following Bernd Schuster's expression of admiration for the 23-year-old prospective world footballer of the year.

After careful consideration, United launched a fierce riposte, claiming the Spanish giants' "unacceptable behaviour" would not be tolerated any longer and have made it clear they will carry through their threat to draw the matter to FIFA's attention if Real do not halt their pursuit.

"The player is on a long-term contract with Manchester United and he is not for sale," A United spokesman said.

"We have watched with growing irritation the comments attributed to Real Madrid over their alleged desire to sign Cristiano Ronaldo. These public attempts to unsettle the player are completely against the regulations and we will not tolerate them any longer.

"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."

Real president Ramon Calderon, however, has claimed his club are not trying to unsettle the player and said: "We haven't interfered in Manchester United's contractual agreement with Cristiano Ronaldo.

"It is a shame that Manchester, a club with which we have excellent relations, are annoyed, because Real Madrid have nothing to do with that aspect."

He added that he felt the issue had been blown out of proportion by the media and he would seek talks with United chief executive David Gill to calm the situation.

Yet the 57-year-old still concluded by saying: "If the team decides to sell the player, that's another thing. What is occurring is a problem between the player and his team."