Cardiff City face a three-window transfer ban unless they pay French club Nantes the first instalment for the transfer of Emiliano Sala, FIFA said on Monday.
FIFA released the full reasoning for their September decision that Cardiff must pay the agreed first instalment of €6 million to Nantes.
After signing for Cardiff for £15m in January, Sala was travelling to Wales from Nantes when his plane crashed in the English Channel.
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The body of the 28-year-old Argentine striker was recovered nearly three weeks later after an underwater search found the wreckage.
FIFA's Players' Status Committee said that Cardiff could face action if they fail to pay the €6m within 45 days of receiving bank details from Nantes.
A statement said: "A ban from registering any new players, either nationally or internationally, for the maximum duration of three entire and consecutive registration periods shall become effective."
The ban for three transfer windows would be lifted if Cardiff complied with the ruling in the meantime.
Cardiff did not immediately respond to a request for comment but the club had previously stated they intend to take the matter to the Court of Arbitration for Sport in Switzerland.
"Cardiff City FC is extremely disappointed at the decision of the Players' Status Committee to award against the club," the Welsh club said after the initial verdict.
"It would appear the committee has reached its conclusion on a narrow aspect of the overall dispute, without considering the full documentation presented by Cardiff City FC to FIFA.
"Nevertheless, there remains clear evidence that the transfer agreement was never completed in accordance with multiple contractual requirements which were requested by Nantes, thereby rendering it null and void."
FIFA's ruling argued that the transfer had been officially completed before Sala's death and therefore Cardiff remained obliged to make agreed payments.
The club, relegated from the Premier League last season, had argued that the circumstances surrounding Sala's death, in a plane organised by an agent who brokered the transfer, meant Nantes were liable for damages caused to them.