Atletico Madrid will be unable to sign new players until January 2018 after the Court of Arbitration for Sport (CAS) on Thursday upheld their transfer ban.
Atletico and city rivals Real Madrid were barred from registering new players for two transfer windows in January 2016 for irregularities in the signing of foreign players under 18.
"This ruling is unfair and causes an irreparable damage to our club," Atletico said in a statement.
Atletico, who appealed the ban in April, are reported to be hoping to sell striker Antoine Griezmann to Manchester United and replace him with another Frenchman, Lyon's Alexandre Lacazette.
Both Atletico and Real saw initial appeals rejected by FIFA last September, although CAS reduced Real's sanction in December, allowing the European champions to recruit new players as of this summer.
Atletico stressed, however, that it had "full confidence in our current squad, with which we have won direct passage to the Champions League for the fifth year in a row and have reached the semi-finals of the Champions League and the King's Cup".
"The club has the commitment of the players and coaching staff to continue competing at the same level next season, despite this decision that is evidently detrimental to us.
"Together, team, club and supporters, we will continue to fight for our goals."
Atletico added: "Our sports planning for next season has contemplated at all times the two scenarios that could arise once CAS announced their decision.
"From this moment on, the club will move forward in the negotiations necessary to formalise the operations to register players from the 1st of January, when we will be authorised by FIFA to register new licenses."