Chelsea hit by new signings ban
Kakuta, 18, joined Chelsea from French side Lens in 2007
Chelsea have been banned from signing any new players until January 2011 by football's governing body Fifa.
The Premier League club was found guilty of inducing French winger Gael Kakuta, 18, to breach his contract with Lens when he joined in 2007.
"Chelsea is banned from registering any new players for the two next registration periods," a statement on Fifa's website read.
Chelsea can appeal against the decision to the Court of Arbitration for Sport.
Fifa's dispute resolution chamber (DRC) ruled Kakuta must pay compensation of 780,000 euros (£682,000), for which Chelsea are "jointly and severally liable".
A spokesman for the London club told BBC Sport they were looking into the matter and hoped to issue a statement shortly.
Lens lodged a complaint with world football's governing body after Chelsea had signed the winger two years ago.
Now the DRC has ruled Kakuta breached his contract with Lens and that Chelsea induced him to do so.
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Fifa's statement continued: "The French club had lodged a claim with Fifa seeking compensation for breach of contract from the player and requesting also sporting sanctions to be imposed on the player and the English club for breach of contract and inducement to breach of contract.
"The DRC found that the player had indeed breached a contract signed with the French club. Equally, the DRC deemed it to be established that the English club induced the player to such breach."
Chelsea must pay additional "training compensation" of 130,000 euros (£114,000) to Lens, while the player has been banned from playing in official matches for four months.
Kakuta ended 2007/8, his first season with Chelsea, as the youth team's top scorer and was voted the academy's player of the year.
His second season was brought to an abrupt end in February when he suffered a double fracture of his ankle.
The next transfer window is between 1-31 January 2010 and the opportunity to register new players after that coming at the end of the current season until 31 August (Europe) and 1 September, (Britain) 2010.
It is a rare punishment by Fifa although another European club - Switzerland's FC Sion - was told in April it could not sign players until the 2010 offseason.
This was punishment for their actions in luring Egypt goalkeeper Essam El Hadary in 2008 before his deal with Al-Ahly had expired.
Like Kakuta, El Hadary received a four-month ban from playing.
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.
Meanwhile Uefa, Europe's governing body, is keen to introduce a rule preventing the international transfer of players under the age of 18 after member associations, clubs, leagues and players agreed to the principle at a March meeting.
After that meeting Uefa president Michel Platini said: "The question of minors is above all a moral and ethical issue. We have a duty to take concrete steps to protect young players and training clubs |