Chelsea say FIFA treated them "differently" to Premier League rivals Manchester City after being cleared to sign players in January as the Court of Arbitration for Sport reduced their transfer ban.
CAS reduced the ban by half after finding Chelsea guilty of breaching two of its regulations on player transfers, specifically regarding bringing in U18s and 'third-party influence' but "for a significantly smaller number of players (about 1/3 of the violations found by FIFA)".
FIFA handed down a two-window transfer ban to Chelsea in February of this year and rejected Chelsea's appeal against the punishment in April.
Chelsea served the first window of their ban in the summer but made a further appeal at the CAS last month, who accepted submissions about the case from the club as well as FIFA.
Premier League champions Manchester City avoided a transfer ban and were fined £315,000 in August for breaches relating to international transfers and registration of players under the age of 18, with FIFA citing "specific circumstances" and "the club's cooperation in clarifying the relevant facts" as factors in the less severe punishment.
A Chelsea statement said: "The approach taken by FIFA to this case has been deeply unsatisfactory, not least as FIFA chose to treat Chelsea entirely differently to Manchester City for reasons that make absolutely no sense to Chelsea.
"Chelsea respects the importance of the work undertaken by FIFA in relation to the protection of minors and has fully cooperated with FIFA throughout its investigation.
"However, if FIFA continues to impose inconsistent and unequal sanctions on clubs then it will not only undermine the very purpose of the regulations, but it will also bring into doubt the game's confidence in FIFA being able to appropriately regulate this important area.
Despite the transfer ban, Mateo Kovacic joined the club from Real Madrid for £40m because of a loophole in FIFA's regulations which allowed the transfer as Chelsea already held his registration after his season-long loan from the previous season.
Despite also losing star player Eden Hazard in the summer, Chelsea started the season well under new head coach Frank Lampard, who has used the punishment to help bring through young players.
They sit in fourth in the Premier League table after 15 games and will be boosted by their ability to strengthen their squad in January.
Chelsea's fine has also been halved from £460,000 to £230,000.
Lampard 'happy' with squad but considering options
Speaking ahead of Chelsea's Premier League clash at Everton, Lampard says the now the club have the chance to sign players again, they must get it right.
"It's a positive outcome for us. The club was working towards a positive outcome for us. It allows us to look at the market going forward," said Lampard.
"I am pleased on a football level. It's obviously a good thing. It wasn't anything to discuss [with the club] at length until we knew the decision."
When asked if Jadon Sancho, Ben Chilwell and Wilfried Zaha were targets he added, "I am not going to sit here and go down too many lines. I am happy with the squad. I am just considering the squad we have got now and in January we can consider our options.
"The fact that people have given us a lot of credit is because people expected less from us. So the players deserve credit. But I want more.
"Clubs below us were spending £100m-£150m and we didn't have that chance. We lost Eden Hazard and lost two number nines but the picture has now changed and we need to get it right moving forward."
Chelsea are free to sign players in January after successfully appealing their transfer ban, but which areas of the squad need strengthening? And which players could Frank Lampard target?
From a Leicester star who could be the solution to their left-back problem to a two big-name attackers who have emerged as potential additions to their forward line, we take a look at what to expect.
How to follow the January transfer window
Sky Sports will bring you the very latest news from the January transfer market with the return of three shows.
Start your day with Good Morning Transfers at 9am as our team of reporters bring you the latest news and insight. Transfer Talk then follows at midday delivering analysis of the biggest stories. Then join us at 7pm for the definitive round-up of the day's news with The Transfer Show.
Meanwhile, the Transfer Talk podcast will also be back at the turn of the year with more expert analysis from here and across the continent.
And as well as tuning into Sky Sports News, don't miss a thing with our dedicated Transfer Centre blog.