The dispute over Emiliano Sala is set to reach FIFA this week if Cardiff fail to comply with Nantes' request for payment of the first instalment of his £15million transfer fee by Wednesday, Press Association Sport understands.
The Argentine striker died when a Piper Malibu plane – piloted by David Ibbotson, who remains missing – crashed into the English Channel on January 21, just two days after Sala signed for Cardiff from the French club.
Officials from both clubs, including Cardiff manager Neil Warnock, attended his funeral in Argentina on Saturday but they have been at odds behind the scenes in a row that is threatening to overshadow what has been a dignified and united response to the tragedy.
Nantes' lawyers wrote to the Premier League side on February 5 asking for the first of three annual payments for the player, as was agreed last month, to be paid within 10 working days.
Cardiff, however, have made it clear they want to wait until the investigation into the crash is complete, as there are several questions still to be answered about the accident, most notably if Ibbotson was even qualified to be flying with a paying customer in that particular plane.
Any wrongdoing in regards to the flight could annul the Welsh club's reported £16million insurance policy for Sala and, if that wrongdoing was another party's responsibility, they may have to sue for compensation.
On the other side, Nantes are equally understandably eager to find a replacement for Sala, their leading scorer for the last three seasons, a situation further complicated by them owing Bordeaux 50 per cent of the fee because of a sell-on clause agreed when they bought him in 2015.
And there is a further twist related to the number of agents involved in the deal, particularly the central role played by Willie McKay, the Monaco-based intermediary who obtained the mandate from Nantes to sell Sala to a Premier League club.
McKay has told Press Association Sport he believes Cardiff are "trying to throw him under the bus" for Sala's tragic death and claims the club are not being honest about what they knew of his close involvement in the transfer.
He claims he personally paid for six similar flights to help get the deal over line, including two for Warnock to scout the player in France and Sala's flights to visit Cardiff for the first time and then to return to Wales to sign his contract.
Press Association Sport has contacted Cardiff for a response to these claims but the club has not responded.
McKay also confirmed he was aware of Nantes' intention to escalate its pursuit of the transfer fee by taking its case to FIFA, where the most likely response is to send the matter to its dispute resolution chamber (DRC), the body that arbitrates when players or clubs have a disagreement. Its chairman is former Football Association chairman and FIFA vice-president Geoff Thompson.
With the losing party, however, likely to be aggrieved, it is possible both sides would agree to skip a stage in the process and go straight to the Court of Arbitration for Sport in Lausanne.