BBC Sport's James Pearce explains the Kakuta ruling
Chelsea could be allowed to sign new players in the January 2010 transfer window despite a Fifa ban.
Football's governing body handed out a ban until 2011 after they found the Blues guilty of inducing Gael Kakuta to break his contract with Lens in 2007.
But Chelsea could get a reprieve if they delay lodging any appeal to the Court of Arbitration for Sport (Cas).
Cas has indicated that if Chelsea take the full 21 days to appeal, their ban may be frozen while a decision is made.
"It depends on the filing date of the appeal," Cas secretary general Matthieu Reeb told BBC Sport.
"If we have an appeal within the next week, it seems realistic we will have a hearing in November and a decision in December.
"If it goes a little longer, we'd have to wait until next year but then we'd have a problem with the January transfer period.
"So there is a possibility of Chelsea requesting a stay of the Fifa decision so it is frozen until the final decision of Cas is rendered.
"This means the enforcement of the Fifa decision would not be immediate. It could be postponed and then if the sanction was finally confirmed by Cas, the suspension would be postponed to the next transfer window."
As it stands, Chelsea are barred from registering any new players, nationally, or internationally, during the January or summer 2010 transfer windows.
The Blues have already confirmed that they will launch the "strongest possible appeal" and they have 21 days to do so from when they receive Fifa's full decision.
The Cas decision can be appealed only before the Swiss Supreme Court.
Manchester United could also face an investigation after it emerged that another French club, Le Havre, are going to ask Fifa to look into Paul Pogba's move to Old Trafford last month.
Fifa say they are yet to receive any complaint over the transfer of the 16-year-old midfielder.
United told BBC Sport last month that Le Havre's accusations that they had stolen the French youngster were "complete nonsense".
A French football agent, who wishes to remain anonymous for professional reasons, told BBC Sport: "If Lens or Le Havre were legally able to sign the two players to contracts they would definitely have done it already.
"But French law prevents players from signing contracts if they under 16 and haven't achieved the requisite level at school.
"The clubs can make agreements with these players that say they will give them a contract when they have attained the right educational qualification or turned 16.
"It is a pre-contract. It is called an 'accord de non sollicitation' in French. It is a written agreement between club and player.
"That is what Kakuta and Pogba had. Fifa is clearly now saying these accords are as good as contracts, while Chelsea and Manchester United are saying they are not."
Fifa punished Switzerland's FC Sion for a similar offence to Chelsea's in April and the club was told it could not sign players until the 2010 off-season.
Sion have appealed to Cas, which has frozen the sanctions while it considers the case, allowing the club to trade before the current season began. A ruling is expected later this year.
Italian club Roma were banned from signing players for two transfer windows in 2004, over their move for Auxerre's Philippe Mexes, after they appealed to Cas and their suspension was reduced to one window.
The arbitration panel will be made up by one judge selected by Chelsea, one by Lens and the chairman will be from Cas.
French Football Federation president Jean-Pierre Escalettes, meanwhile, is pleased with the decision to punish Chelsea.
He said: "We had alerted Fifa and the Football Association against such misbehaviour. We are perfectly happy to see it being sanctioned."
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