United are incensed at what they see is a deliberate campaign to unsettle the 23-year-old Portugal winger, currently on duty at Euro 2008.

A FIFA spokesman told PA Sport: "I can confirm that we have received an official request from Manchester United to look into the matter.

"We have only just received the documents so we need some time to analyse them before we can make any decision."

FIFA's regulations do not stipulate any possible sanction against Real Madrid but they do say one club have to inform another if they are to start negotiations with one of their players.

The rules do not specifically state, however, that the club that owns the player have to give consent.

Article 18 of FIFA's regulations for the status and transfer of players states: "A club intending to conclude a contract with a professional must inform the player's current club in writing before entering into negotiations with him.

"A professional shall only be free to conclude a contract with another club if his contract with his present club has expired or is due to expire within six months. Any breach of this provision shall be subject to appropriate sanctions."

United manager Sir Alex Ferguson has already voiced his dislike of Real's touting for Ronaldo, claiming their major domestic rivals, Barcelona, refrain from public expressions of admiration for other clubs' players.

He is adamant Ronaldo neither wants to, nor will be, leaving Old Trafford this summer.

A statement on the club's official website, www.manutd.com, read: "On 27 May, Manchester United made clear its 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."

On the date stated, coach Bernd Schuster said Ronaldo was his club's number one summer transfer target.

The Portugal winger scored 42 goals last season and was named Player of the Year by both the Professional Footballers' Association and the Football Writers' Association.

Schuster is an admirer of arguably the world's finest player and said: "There are other players we have been looking at but everything has slowed down a little due to all this Cristiano Ronaldo business, seeing as he is the big priority for the club.

"The English way has been a perfect education for a player with such exceptional individual talent.

"There, he has learned to enjoy his football as a complete professional. This means we would get a Cristiano who would arrive here with his lessons learned."

Ronaldo has continued to send out mixed messages about his future after helping United win the Champions League and retain their Barclays Premier League title.

He recently told Brazilian website Terra: "I would like to play for Real Madrid - but only if it is true they are ready to pay what Manchester United ask of them. However, it does not depend on me."

He then vowed not to discuss the subject further until after Euro 2008.

A Real spokesperson said: "We know nothing of this matter at this time and we have no comment to make on the topic."