West Ham winger Said Benrahma saw a move to Lyon collapse late on Deadline Day.
The French club have been left angry their deal to sign the forward is off, believing the Hammers did not enter the required data into FIFA's Transfer Matching System (TMS).
"Olympique Lyonnais was delighted to welcome Saïd Benrahma to Lyon this Thursday to finalize his transfer from West Ham," read a statement from the French club.
"The two clubs had signed a temporary transfer agreement, and Olympique Lyonnais had obtained the approval of the DNCG to allow the transfer to take place in a fluid and transparent manner.
"However, in the early evening, when Olympique Lyonnais had entered all the administrative data on FIFA's dedicated platform (FIFA TMS), West Ham had never launched any technical procedure on its part, despite OL's repeated reminders and continuous mutual telephone communication.
"In the absence of this reciprocal action on the part of the English club and following this incomprehensible behavior, raising questions, the international transfer certificate could not be requested before closing time.
"Olympique Lyonnais deeply regrets this situation and this decision, demonstrating a deep lack of respect on the part of West Ham for the Institution and the player.
"The club reserves the right to initiate all appropriate procedures necessary to validate the operation later, and to hold West Ham accountable if necessary."
Benrahma had completed a medical and was set to join the Ligue 1 outfit on loan ahead of a permanent transfer worth around £17m this summer.
A West Ham spokesperson said: "West Ham United can confirm that the Deadline Day deal for Saïd Benrahma to join Olympique Lyonnais was not concluded ahead of the deadline, with a deal unable to be completed before 11pm."
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Benrahma signed for West Ham from Brentford in 2020 on loan, before that move was made permanent just a few months later for a fee rising to £30m.
He has started only five Premier League matches this season for West Ham, and has not scored a single goal in any competition.
The Algerian's final act for the Hammers was to get sent off for kicking out in the FA Cup replay against Bristol City last month, as David Moyes' side were knocked out by the Championship outfit.
