Chelsea have been banned from signing players for the next two transfer windows after FIFA found them guilty of breaching rules related to the signing of players under the age of 18.
It means Chelsea, who signed Christian Pulisic from Borussia Dortmund in January and loaned him back until the summer, cannot bring in players until summer 2020, but it does not affect their ability to release players or bring back loan players.
The club have said they will appeal the decision.
FIFA said Chelsea had been found to have "breached article 19 of its regulations in the case of 29 minor players and to have committed several other infringements relating to registration requirements for players."
It added that the club had also "breached article 18bis of the Regulations in connection with two agreements it concluded concerning minors and which allowed it to influence other clubs in transfer-related matters."
The club have also been fined £460,000 ($599,000) and given 90 days to "regularise the situations of the players concerned."
Last month, it emerged that FIFA's investigation into Chelsea's transfer dealings had widened to examine arrangements with more than 100 foreign players under the age of 18.
However, the club said many of the individual cases highlighted involved players on short-term trials who were not signed.
In a statement on their website, Chelsea said they "acted in accordance with the relevant regulations" and were "extremely disappointed" by FIFA's decision.
"Chelsea FC has today received a decision from the FIFA disciplinary committee concerning alleged breaches of the FIFA regulations that relate to the international transfer of players under the age of 18," the statement said. "Chelsea FC categorically refutes the findings of the FIFA disciplinary committee and will therefore be appealing the decision.
"The club wishes to emphasise that it respects the important work undertaken by FIFA in relation to the protection of minors and has fully cooperated with FIFA throughout its investigation. Chelsea FC acted in accordance with the relevant regulations and will shortly be submitting its appeal to FIFA."
FIFA transfer regulations prohibit clubs from signing under-18 players from other countries unless their parents have emigrated for reasons not connected to football or both the player and club are based within 50 kilometres of a national border.
Chelsea's academy has been under investigation for potential transfer rule breaches since 2016, when pictures emerged of Bertrand Traore playing against Arsenal in a noncompetitive match as a 16-year-old.
The club said they had approval from both the Football Association and Premier League to play Traore in trial games.
Traore left Stamford Bridge to join Lyon for £8.8 million in 2017, although it is understood that Chelsea retain the right to buy him back for an agreed price.
FIFA has previously investigated Real Madrid, Barcelona and Atletico Madrid for breaching rules relating to the signing of players under 18.
Each club was given a two-window transfer ban, though Real Madrid's punishment was reduced to one window on appeal to the Court of Arbitration for Sport.
FIFA also found the Football Association in breach of its transfer regulations, fining it £390,000 and giving it "a period of six months to address the situation concerning the international transfer and first registration of minors in football."
An FA spokesman said the organisation intended to appeal against the FIFA punishment and would "continue to work with FIFA and Chelsea in a constructive manner to address the issues which are raised by this case."