Barcelona has already been hit with a transfer ban
after breaching regulations when signing young players, but rival Real insists it has complied fully with FIFA's rules when conducting its own under-age deals.
Real said it has shown "extraordinary diligence" during the transfer and registration of the 51 cases of foreign minors that are being investigated.
The Spanish club issued a lengthy statement
confirming the governing body's request for documents relating to the various deals taking place over the last five years, though it did not name any of the players.
"Real Madrid defends the protection and the correct and healthy development of minors via its strict adherence, without exceptions, to the Regulations on the Status and Transfer of Players, and is in complete agreement with such investigations being carried out to this end," the statement read.
Article 19 of FIFA's regulations
prohibits the transfer of players under the age of 18, unless they meet one of three qualifying criteria.
Under-age players can move to a club in a different country if their parents move there for non-footballing reasons, if they are from another nation within the European Union or European Economic Area and aged between 16 and 18, or if they live within 100km of the club.
Real provided explanations for each of the 51 cases, stating that the majority of players were exempt from regulations for various different reasons.
Among the exemptions it provided were that 10 players were Spanish nationals, 23 foreign players had previously been registered with other Spanish clubs, six players were 18 or over at the time of their registration, two players had never belonged to Madrid and one player appeared twice on FIFA's list. Real also offered up further explanations for the remaining nine cases.
"Real Madrid has responded diligently, supplying all the information required by the Committee within the established deadlines and in complete documentary detail," added the Real statement.
"Both Real Madrid and the RFEF [Royal Spanish Football Federation], through which the informative investigation is being processed, have been left totally satisfied with the information supplied and have verified the strict adherence to the registration regulations in absolutely every case."
Madrid director Emilio Butragueno had earlier confirmed Monday that FIFA is investigating the club, but is confident there is nothing to worry about.
"We are in agreement with FIFA in this matter and we will continue collaborating with them in everything that is asked of us," he told reporters. "We are absolutely calm about how Real Madrid has proceeded."