FIFA has refused to accept Leicester's documentation in support of registering new signing Adrien Silva, according to Sky sources.
The Foxes agreed a £22m deal for the 28-year-old Portugal international midfielder with Sporting Lisbon on Deadline Day, but had to request a two-hour extension to complete the necessary paperwork.
It is understood Leicester's officials are working with the Portuguese club to try to correct the registration papers, so that the transfer may yet be sanctioned by FIFA.
A Leicester City spokesman said: "We are working with Adrien and Sporting Clube de Portugal to overcome some issues relating to the player's registration and exploring all options to find a resolution."
It will be hugely frustrating for the club and midfielder who missed Portugal's 5-1 World Cup qualifying win against the Faroe Islands on Deadline Day and remained in England hoping for a swift resolution.
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Leicester targeted the 28-year-old as a replacement for Danny Drinkwater, who was sold to Chelsea for £35m in the final hours of the summer transfer window.
Drinkwater was announced as a Stamford Bridge arrival at 1.30am on September 1 but because Silva was an international transfer, Leicester had to await clearance to announce the deal.
Sporting said at the time in a statement: "Sporting Club de Portugal... informs the market that we have reached an agreement with Leicester for the definitive transfer of the sporting and economic rights of the professional soccer player Adrien Silva. The registration of the same (player) is awaiting confirmation of FIFA."
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Silva is a product of Sporting's academy and has had loan spells at Maccabi Haifa and Academica. He has earned 20 caps for Portugal, despite being born in France.
He is a Euro 2016 winner with Portugal and has scored 32 goals in 136 league appearances during his six years with Sporting.