The Foxes were finalising the deal as the August 31 transfer deadline approached.
And now FIFA has said that the 28-year-old Portugal international, will not be allowed to move to the English Premier League after Leicester missed the deadline by seconds.
Silva is not expected to be ineligible for the former league champions until the January transfer window opens, but they will reportedly contest the finding.
A club spokesman was reported on the BBC to have 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.”
Sporting President Bruno de Carvalho added: “The transfer of Adrien is already completed. Hopefully Leicester find a solution for him.”