Fifa has rejected the Football Association’s appeal to rubber-stamp midfielder Adrien Silva’s move to Leicester City from Sporting Lisbon.
Leicester submitted paperwork on Silva’s £22m switch 14 seconds too late on the 31 August transfer deadline.
Fifa withheld Silva’s International Transfer Certificate so the Foxes could not register him to play.
Leicester will now only be able to add Silva to their playing squad when the transfer window reopens in January.
Silva returned to Sporting’s Estadio Jose Alvalade on Sunday and wished his former club’s fans an emotional farewell before their 0-0 draw with rivals Porto.
He has been in the stands to watch Leicester play at King Power Stadium and has worked with fitness coaches at the club’s Belvoir Drive training ground, but is barred from actually working alongside the Leicester squad.
He returned to Portugal last month to be with his family.
Leicester, who are 17th in the Premier League after just one win this season, sold Danny Drinkwater to Chelsea for £35m with Silva earmarked as his replacement.
Instead, Wilfred Ndidi, who signed from Genk last summer, has been paired with Andy King in the centre of midfield in recent matches.
Silva has been left out of Portugal’s squad for their final World Cup qualifiers against Andorra and Switzerland and Foxes coach Craig Shakespeare said last month that the tournament was “in the back of his mind”.