File Photo: Al-Nassr's defender Aymeric Laporte AFP
The Basque side said they tried to sign him on September 1 from Saudi Arabian club Al-Nassr, but it could not be completed for "external factors beyond (Athletic's) control", with Spanish media saying the paperwork was submitted late.
The Spanish federation asked FIFA to grant an exception for Laporte's move but Athletic said they were informed on Wednesday FIFA had denied the request.
Athletic said they were "studying all possibilities within the existing legal framework" to still try and bring in Laporte, but if they could not then the agreements with the player and Al-Nassr will be void.
Spain centre-back Laporte, 31, left Athletic for Manchester City in 2018 before joining Al-Nassr in 2023.
(For more sports news and updates, follow Ahram Online Sports on Twitter at @AO_Sports and on Facebook at AhramOnlineSports.)
Short link: