Cardiff City will tell world football's governing body FIFA that the contract to sign Emiliano Sala from Nantes was not legally binding.
The Welsh club have refused to pay the £15m transfer fee for the striker, who was killed in a plane crash two months ago.
Ligue 1 outfit Nantes have complained to FIFA, and Premier League side Cardiff have until 3 April to respond.
The 28-year-old Argentinian died, along with pilot David Ibbotson, when the Piper Malibu aircraft they were travelling in came down in the English Channel on 21 January, two days after the Welsh club made him their record signing "subject to international clearance".
Cardiff have refused to pay the first £5m instalment of the Sala transfer fee.
It is understood Cardiff will argue that Sala's contract had been rejected by the Premier League because it contravened signing-on fee rules and was therefore 'null and void'.
He had been unable to sign a revised deal before his death.
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The club will also claim that further contract clauses – proposed by Nantes – had not been met.
Without these clauses being fulfilled, Cardiff will argue Sala was perfectly entitled to join another club with more than a week of the January transfer market remaining.
Nantes say they are fully compliant with FIFA rules and point to the fact that FIFA registered the international transfer certificate on 21 January, 2019 at 5.30pm.
A Cardiff spokesperson said: "The club is aware of FIFA's request for a response by 3 April and is processing that accordingly. We have no further comment at this stage."
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