Chelsea have lodged an official appeal with the Court of Arbitration for Sport (CAS) against the two-window transfer ban issued by FIFA.
The Premier League club were initially sanctioned in February as punishment for the transfer of young foreign players.
A subsequent appeal to governing body FIFA was rejected last month and the Europa League champions have now formally taken their case to CAS.
"The Court of Arbitration for Sport (CAS) has registered an appeal filed by Chelsea Football Club Ltd (CFC) against the Federation Internationale de Football Association (FIFA)," read a statement.
The ban prohibits Chelsea from signing players during the 2019 summer window – which closes on August 8 – and in January 2020.
A panel of arbitrators will now deliberate the appeal but CAS says "it is not possible to say at this time" when a decision will be made.
"The appeal is directed against the FIFA Appeal Committee decision dated 11 April 2019 in which CFC was declared liable for violations of the FIFA Regulations on the Status and Transfer of Players and banned from registering new players, nationally and internationally, for two entire and consecutive registration periods," continued the statement.
"Following the hearing, if one is held (a CAS panel may also issue a decision based only on the parties' written submissions), the panel deliberates and then issues its decision in the form of an Arbitral Award.
"It is not possible to say at this time when the Arbitral Award will be issued."
The London club were also fined 600,000 Swiss francs (over £460,000) for the breach of regulations, which related to the recruitment of 29 players aged under 18.
Blues boss Maurizio Sarri has since said the club need to overturn the suspension in order to compete for the title next season.
However, Sarri's own future has become increasingly unclear, adding to the uncertainty at Stamford Bridge.
The Italian, who was recruited from Napoli last summer to replace compatriot Antonio Conte, has been heavily linked with a return to Serie A and the vacant managerial role at Juventus.
Chelsea also look set to lose star player Eden Hazard to Real Madrid this summer, with a deal reportedly agreed between the two clubs and the player considering a new challenge.
Should the appeal to CAS be rejected, the club would be unable to sign an immediate replacement for the Belgium forward.
Their squad, however, will be strengthened by the arrival of United States winger Christian Pulisic, who was loaned back to Bundesliga runners-up Borussia Dortmund after a £58million transfer was completed in January.
Chelsea's options could be further reinforced by the return of numerous other first-team players loaned out last season.
While Spain forward Alvaro Morata has suggested he would like to stay with Atletico Madrid, the likes of defender Kurt Zouma, midfielder Tiemoue Bakayoko, winger Victor Moses, and strikers Tammy Abraham and Michy Batshuayi may have futures in west London.
