The Lokomotiv Moscow midfielder appeared set to join the newly-promoted Londoners, having undergone a medical over the weekend.
Harry Redknapp even appeared to confirm the deal following QPR’s 1-0 win over Sunderland on Saturday, telling reporters: “Lassana Diarra has signed and I’m delighted to have him."
Yet the deal has now fallen through on transfer deadline day, despite Diarra reportedly snubbing a late approach from Roma in order to move to England.
It is understood there was confusion over whether Diarra was a free agent and whether a transfer fee would have to be paid to acquire his signature, though the official reason for the collapse of the deal remains unexplained.
The development may see QPR redouble their efforts to sign Tottenham midfielder Sandro, with the club hopeful of completing a loan deal for the Brazilian.