FIFA has banned two of the world's biggest football teams from signing players

Two of the world's biggest football teams have been banned from signing new players after breaking rules regarding the signing of footballers who haven't yet turned 18.

Real Madrid, and Atletico Madrid, the two biggest teams in Spain's capital had their transfer privileges taken away until the summer of 2017 by football's governing body FIFA, and will not be able to buy or register any new players until the end of next season.

The ban centres on both clubs separately violating rules designed to protect child stars from being exploited, and to ensure that they are protected in one of the world's most high pressure sports. Both teams have also been fined, with Atletico stung by a 900,000 Swiss franc (£622,000; $895,000) fine, and Real given a 360,000 Swiss franc (£249,000: $358,000) bill.

Commenting on the ban on Thursday night, a FIFA statement said:

"FIFA works hard to protect the rights of players under the age of 18 - whether male or female, amateur or professional. This is done through the enforcement of regulations prohibiting the international transfer of minors, or the first registration of minors in a country other than their own, except in specific circumstances.

The two clubs were found to have violated several provisions concerning the international transfer and first registration of minor players, as well as other relevant provisions with regard to the registration and participation of certain players in competitions."

FIFA didn't say exactly what the clubs had done wrong, but, as reported by the Independent, they are likely to have broken one of three key rules surrounding junior players signing for big clubs. Under the rules, players can only be signed if:

1. The players' parents have moved to the city where his new club is based, but not because he is moving teams.
2. The transfer is between two European clubs, with the player older than 16.
3. A player lives 50km from the border of the country he is moving to and the club is 50km from the same border.

The ban could be a big problem for both clubs who rely heavily on recruiting expensive new stars. This is especially true in the case of Real Madrid, which is famous for signing huge stars, and for breaking the world record for the most expensive player several times.

Most recently, the signings of Gareth Bale in 2014 and Cristiano Ronaldo in 2009, for £85 million ($122 million) and £80 million ($115 million) broke the world transfer record.

Both clubs have said they plan to appeal the ban, with Real Madrid calling it "absolutely unacceptable" in a statement. Atletico were slightly more sanguine: "Our club does not agree with the sanctioning ruling from the international football body and is studying the documentation received to lodge an appeal," a statement from the club said.

Under the ban, both teams will still be able to sell current players.

Real and Atletico aren't the only Spanish clubs who've been hit with a transfer ban recently. In the summer of 2014, big rival FC Barcelona was handed an almost identical ban, after also breaking FIFA's rules about signing players under the age of 18. Barcelona's ban has just been lifted.