Chelsea winger Hakim Ziyech's proposed loan move to Paris St-Germain has been blocked by France's governing body, the LFP.
A deal was agreed before the French deadline for the 29-year-old.
But PSG sources were critical of Chelsea's conduct, claiming required paperwork was not submitted on time.
Ziyech was in Paris expecting a deal to be confirmed, but the registration was not completed and so he will return to Chelsea for the rest of the season.
BBC Sport understands the player contacted Chelsea owner Todd Boehly directly in an effort to resolve the matter, but his pleas were ultimately unsuccessful.
The issues are thought to have involved legal agreements between the two clubs, which were in order and just needed signing.
Chelsea sent theirs back by the deadline, but due to a technical issue PSG did not receive it.
PSG requested a copy which was sent back at 23:00 GMT, by which time it was too late for Ziyech to be registered.
The LFP dismissed PSG's appeal on Wednesday morning.
Ziyech was the Blues' first signing following their transfer ban in 2020, arriving from Ajax for £33m.
However, they have signed forwards Mykhailo Mudryk, David Datro Fofana, Noni Madueke and Joao Felix this month, in addition to England's Raheem Sterling who joined last summer.
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