FIFA has rejected Leicester City's request to ratify the signing of Adrien Silva from Sporting CP.
The 2015-16 Premier League champions agreed a deal worth up to EUR29.5million to sign the Portugal international as a replacement for Danny Drinkwater, who joined Chelsea shortly before the close of the transfer window.
Leicester reportedly missed the deadline to register Silva by a matter of seconds, meaning FIFA refused permission for the move to be completed.
The club lodged an appeal against the decision with the assistance of the Football Association, but it has been turned down.
A FIFA spokesperson told Omnisport: "We can confirm the following decision of the Single Judge of the Players' Status Committee: the petition made by The Football Association for permission to request the International Transfer Certificate for the player Adrien Silva and subsequently register the player for its affiliated club, Leicester City FC, is rejected."
It means an uncertain few months for Silva, who will be unable to join Leicester until the start of the next transfer window in January.
The 28-year-old has been socialising with his prospective new team-mates but has not been allowed to train with them due to FIFA's ruling.
Craig Shakespeare's side are just one place outside the relegation zone, having won only one of their first seven league games.