Spanish champions Barcelona have been given a 14-month transfer ban by Fifa for breaking rules on signing international players under 18.
The Catalan club cannot buy or sell players until the summer of 2015 after the world governing body imposed a transfer ban for the next two windows.
They have also been fined 450,000 Swiss Francs (£305,000).
"This is a shattering piece of news for a club that had already endured a very difficult season off the pitch following allegations of tax evasion around the signing of Neymar.
"That saga led to the resignation of controversial club president Sandro Rosell in January. Now his replacement, Josep Maria Bartomeu, can expect overwhelming pressure from fans to call new presidential elections as soon as possible.
"The impact upon the team is also hugely significant. With veteran captain Carles Puyol and goalkeeper Victor Valdes leaving at the end of the season, the club had already announced their intention to splash out on top-class replacements. Now that seems impossible, which means the squad will be severely weakened next season.
"It is also yet another massive blow to Barca's previously much-respected global brand, which has taken a huge battering in the last few months and will be further tarnished by this latest development."
The Spanish Football Federation (RFEF) has been sanctioned by Fifa for the same breaches.
Barcelona are hoping to sign a new goalkeeper at the end of the season, with the long-serving Victor Valdes leaving the Nou Camp at the end of his contract.
The four-time European champions have been linked with Borussia Monchengladbach keeper Marc-Andre ter Stegen, while they agreed a deal to sign 17-year-old Croatia midfielder Alen Halilovic last week.
The RFEF has been given a fine of 500,000 Swiss Francs (£340,000) and told to "regularise its regulatory framework and existing system concerning the international transfer of minors in football" within a year.
Fifa rules state that international transfers are only permitted for players over the age of 18 - unless the player in question meets one of three qualifying criteria.
Under-18s 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.
A Fifa investigation - centred on several players aged under 18 who were registered and played for the club between 2009 and 2013 - found that Barcelona and the RFEF were guilty of a "serious" infringement of the rules in relation to 10 players.
Fifa told BBC Sport the identity of these players was confidential.
Its disciplinary committee said it sought to protect the best interests of young players, even if an international transfer might be beneficial to their football careers.
"On the basis of this analysis, the committee concluded that 'the interest in protecting the appropriate and healthy development of a minor as a whole must prevail over purely sporting interests'," said a Fifa statement.
It is the latest off-the-field issue to trouble Barcelona this season.
Earlier this year, the Catalan club were accused by the Spanish authorities of tax evasion relating to the £50m purchase of Brazil forward Neymar. A criminal investigation is under way, despite Barca making a "voluntary contribution" of more than £11m in taxes.
The club's president Sandro Rosell resigned after he was accused of misappropriating funds in the deal.
Rosell and his successor Josep Maria Bartomeu insist the 22-time Spanish champions have not acted illegally.