Lyon accused West Ham of “incomprehensible behaviour” and a “lack of respect” after the deal to take Said Benrahma to France collapsed late on Deadline Day.
Benrahma travelled to Lyon in a bid to complete the move before the French window closed at 10pm.
The deal was a loan with an obligation to buy – in total worth more than £15m.
However, despite Lyon claiming the deal was agreed, the move collapsed at the last minute with the Ligue 1 side blaming West Ham for the breakdown.
A statement released by the French club said: “Olympique Lyonnais were delighted to welcome Said Benrahma to Lyon this Thursday to finalize his transfer from West Ham. The two clubs had signed a temporary transfer agreement, and Olympique Lyonnais had obtained approval from the DNCG to allow the transfer to take place smoothly and transparently.
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“However, at the start of the evening, while Olympique Lyonnais had entered all the administrative data on the dedicated FIFA platform (FIFA TMS), West Ham had never launched the slightest technical procedure on its part, despite the repeated reminders from the OL and continuous mutual telephone communication.
“In the absence of this reciprocal action on the part of the English club and following this incomprehensible behaviour, raising questions, the international transfer certificate could not be requested before closing time.
“Olympique Lyonnais deeply regrets this situation and this decision, demonstrating a profound lack of respect on the part of West Ham towards the institution and the player.
“The club reserves the right to initiate all appropriate procedures necessary to validate the transaction at a later date, and to hold West Ham responsible if necessary.”
West Ham responded with a statement of their own: “West Ham United can confirm that the Deadline Day deal for Said Benrahma to join Olympique Lyonnais was not concluded ahead of the deadline, with a deal unable to be completed before 11pm.”
Confusion surrounds Fornals deal to Real Betis
Real Betis are still waiting to find out if the deal to sign West Ham midfielder Pablo Fornals has been finalised.
All parties believe the correct documentation was processed in time. Deal is a permanent transfer worth close to £7m.
Clarity on whether the deal has gone through is expected on Friday.
Fornals was at the London Stadium alongside his West Ham team-mates on Thursday, watching on as they drew 1-1 with Bournemouth.