Madrid clubs lose battle against transfer bans
The decision means both clubs, which denied any wrongdoing, will be prevented from registering any players at national and international level during the next two transfer windows.
The teams can make deals to sign players, but cannot register them to play.
FIFA's disciplinary committee handed out the punishment in January for breaches of regulations governing the transfer of players under the age of 18. The bans were later frozen pending an appeal.
A statement from FIFA said: "The FIFA Appeal Committee has decided to reject the appeals lodged by Spanish clubs Atletico de Madrid and Real Madrid and to confirm in their entirety the decisions rendered by the FIFA Disciplinary Committee in the respective cases relating to the protection of minors.
"As such, both clubs are to serve a transfer ban that prevents them from registering any players at national and international level for the next two complete and consecutive registration periods for breaching articles 5, 9, 19 and 19bis as well as annexes 2 and 3 of the Regulations on the Status and Transfer of Players (the "Regulations")."
FIFA's rules, which ban international transfers of under-18 players except in certain circumstances, exist to "protect minors who move to other countries and prevent soccer clubs from exploiting them."
"Although FIFA's approach may appear harsh in an individual case, it is only by enforcing the rules consistently and strictly that the abuses of the past can be avoided and a proper and safe development of minor players secured," FIFA added.
Real has hit out at the decision, issuing a strongly worded statement on its official website which stated it will take its case to the Court of Arbitration for Sport (CAS).
The statement read: "In light of today's decision by the FIFA Appeal Committee, Real Madrid C. F. wishes to communicate the following:
1. The club regrets said decision, understanding it to be deeply unjust and contrary to the fundamental principles of disciplinary law.
2. The club will begin the appropriate appeals procedure before the Court of Arbitration for Sport (CAS), requesting that the decision be completely overturned, with absolute confidence that said organisation will come to an entirely favourable decision.
3. Real Madrid C. F. will request that the procedure be undertaken in an urgent manner in order to obtain a decision from the CAS as quickly as possible."
Atletico has also confirmed it will challenge the verdict after releasing the following statement:
"Our club does not agree with the ruling, so it has decided to appeal to the Court of Arbitration for Sport (CAS)," it read.
"Anticipating that FIFA could ratify this ban, the club hired months ago a team of international experts to defend the case and protect the rights of Atletico de Madrid.
"Our organization wishes to express that 100 percent of the licenses of our players that are part of this investigation and are still in the club, have been regularized by FIFA, receiving the authorization from the subcommittee in order to process their licenses and compete in their different categories."
Real and Atletico aren't the only clubs to have fallen foul of FIFA's rules about signing players under the age of 18.
In the summer of 2014 Barcelona was handed an almost identical ban for the same offense, while Chelsea was banned in 2009 from trading in two transfer windows over a dispute with Lens over the signing of Gael Kakuta.
However, CAS later overturned the decision against the English club.