Cardiff will appeal to the Court of Arbitration for Sport against FIFA's decision to make them pay Nantes the first instalment of the transfer fee for Emiliano Sala.
The Argentinian, 28, died when the light aircraft he was travelling in from France to the UK crashed into the English Channel on January 21 this year, two days after the Bluebirds, then in the Premier League, had announced his signing.
World governing body FIFA on Monday ordered that Cardiff should pay the first instalment of the overall £15million transfer fee, equating to six million euros (£5.3million) – something the Welsh club contest on the grounds that Sala's revised contract was not signed and that his playing registration was not complete.
"Cardiff City FC is extremely disappointed at the decision of the (FIFA) Players' Status Committee to award against the club," the Championship club said in a statement.
"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 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.
"We shall be appealing to CAS in order to seek a decision which considers all of the relevant contractual information and provides clarity on the full legal situation between our two clubs.
"This is a complex matter, which includes ongoing civil and criminal consideration both in the UK and abroad, which will likely have an impact on the validity of the transfer. It is therefore vital that a comprehensive judgement is reached following a full assessment and review of the facts."