Atletico Madrid will not be able to sign new players this summer as their transfer ban has been upheld by the Court of Arbitration for Sport.
The Spanish side were banned in July 2016 from registering players for two transfer windows for breaching Fifa rules over the signing of minors.
"This ruling is unfair and causes an irreparable damage to our club," said Atletico in a statement.
In December, Real Madrid had their ban for the same offence halved on appeal.
Atletico added that the decision amounts to "comparative disadvantage and discriminatory treatment" of the club, given rivals Real will now be able to sign players in the transfer window that starts on 1 July.
Cas have reduced Atletico's fine from 900,000 Swiss Francs (£719,793) to 550,000 (£439,873).
Champions League semi-finalists Atletico said they had "full confidence" in their current squad and were determined to continue "competing at the same level next season", having finished third behind Real and Barcelona in La Liga.
Atletico forward Antoine Griezmann, 26, has been linked with a move to Manchester United this summer., but BBC Sport understands the Red Devils have now cooled their interest in the France international.
Fifa's investigation concerned players aged under 18 who played in competitions for Atletico between 2007 and 2014, and Real from 2005-2014.
The world governing body initially ruled on the case in January 2016, but both clubs were able to sign players last summer after appealing against the decision to Fifa.
The two sides then went to Cas after those appeals were rejected, with Real Madrid's suspension subsequently reduced to cover just the recent January transfer window.
Cas' decision was due to be released on Wednesday but was delayed until Thursday, which Altetico said showed a "lack of respect" to the club.
Atletico will be allowed to register new players again from 1 January 2018.