Emiliano Sala signed a potentially invalid contract with Cardiff City in the days before he died in a plane crash, the Daily Telegraph can reveal.

Lawyers working on behalf of England's top tier have written to the Welsh club this week confirming their record signing was not fully registered to play in the competition last month.

The remarkable latest twist further muddies a legal row over whether Cardiff should pay Nantes the full £15million transfer. Sala's problematic contract came to light after Cardiff sought clarification as part of their attempt to stave off threatened legal action from Nantes over the freezing of payments.

The Telegraph understands Sala’s Cardiff contract was deemed non-compliant with Premier League rules and was returned to them for him to sign an amended one. Neither the club nor the league would comment last night on the nature of the documentary discrepancies, although it is understood such issues are not uncommon.

Cardiff plan to use Sala’s Premier League registration status to argue he was not their player when he died almost a month ago. They have also written to Nantes seeking evidence the French club had fully deregistered the Argentinian prior to the January 21 crash.

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As of Wednesday, Nantes had yet to carry out threats issued earlier this month of legal action against Cardiff if the latter did not pay the first instalment of Sala’s transfer fee.

The Welsh club are continuing to build a negligence case against their French counterparts over the plane crash and are understood to want Sala’s family to join them. More than one source said their case had been strengthened by an explosive interview given by Willie McKay to the Telegraph this week in which he confirmed his agent son, Mark, had been contracted to work for Nantes.

The McKays, who helped broker Sala’s deal, arranged his trips to Cardiff and back during transfer talks and the club plan to hold Nantes vicariously liable as a result if an official report finds any flight regulations were breached on January 21. That could include rules governing safety equipment.

Relations between Cardiff and McKay Snr have deteriorated to such an extent that the club have written to him warning they reserve the right to deny him entry to their ground.

Cardiff have also snubbed a request by him for a public meeting about the Sala saga with owner Vincent Tan, chairman Mehmet Dalman, chief executive Ken Choo and manager Neil Warnock.

Cardiff City manager Neil Warnock attending Sala's funeral in Santa Fe, Argentina credit: AP Photo/Natacha Pisarenko

On Wednesday, both Nantes and the McKays found themselves at the centre of unrelated tax wrangles. Inspectors searched club premises at Nantes on Tuesday as an evasion investigation was launched by the National Public Prosecutor against the club's president, Waldemar Kita. Nantes were not immediately able to comment.

Meanwhile, a High Court hearing in London heard how Mercato Sports Ltd, a business registered in the names of Mark McKay and Willie McKay's wife, had cleared a tax debt in a last-gasp bid to fend of an HM Revenue and Customs winding up order.

The McKay family business was mentioned briefly at the Insolvency and Bankruptcy Court at the Rolls Building in central London. A barrister for HMRC told the court: “The debt has been paid. The petition is dismissed with costs.”

Sala, whose body was recovered from plane wreckage two weeks ago, died from “head and trunk injuries”. The pilot, David Ibbotson, is still missing but searches are set to be resumed for his body.