United said in a statement: "On May 27 [we] made clear [our] intention to report Real Madrid to Fifa if it continued to involve itself in the future of Cristiano Ronaldo.
"Unfortunately, Real Madrid has not kept its own counsel and the club feels it has no alternative but to make a formal complaint to the world governing body, which it has done today. The precise content and nature will remain confidential."
Despite the steps United have taken, lawyers do not believe Real are likely to be severely reprimanded over their conduct and a censure, rather than anything like a points deduction, seems most likely.
United's move demonstrates their anger in a position of apparent helplessness. Sir Alex Ferguson said at the weekend that he does not intend to travel to Portugal to discuss the situation with Ronaldo but it is the winger's conduct just as much as Real's which has contributed to the escalating uncertainty.
If further evidence were needed of his indifference to a future at United it came yesterday when the player's two siblings Hugo and Elma Santos Aveiro said they thought he would like to leave. "Between now and next year, or the year after he will go to Real Madrid," Hugo said. "Personally, I would like that. I'd like it very much." Elma said: "He won't decide anything until after the Euros."
United have preferred to characterise the situation as one of Real's making, pointing out that the club's Catalan rivals in La Liga, Barcelona, generally refrain from public expressions of admiration for other clubs' players.
The club are indignant about suggestions that they might be prepared to negotiate a new and superior contract for Ronaldo, but an acknowledgement from within Old Trafford that he might be keen to go finally came from the club's former captain Bryan Robson, who is now an ambassador for United.
"If he weighs everything up, then why move if he is happy where is is?" Robson said. "Money is not everything. I cannot see much of a temptation to go to Real. He is playing fantastic football and is admired by all the fans, players and coaching staff."
Ronaldo is unlikely to make any more pronouncements on his future until the 2008 European Championship in Austria and Switzerland is concluded, leaving United with time to decide whether to allow the player to sit in the stands next season rather than sign for Real, as they have threatened, or to accept that he wants to leave and let him do so.
A spokesman for Fifa, Pekka Odriozola, confirmed the complaint had been received. "We will first have to look at the precise nature of the complaint before seeing what steps will be taken."