Real boss says Man Utd 'lack of experience' killed De Gea deal

And although Real have been chasing the goalkeeper for more than a year as a long-term replacement for Iker Casillas, Perez refused to confirm they would make a second move for de Gea when the transfer window reopens in January.

Madrid seemed set to conclude the long-running transfer saga late on Monday's deadline day with a deal worth between 30 million ($33.9 million) and 40 million euros and Costa Rica goalkeeper Keylor Navas joining United.

But Spanish league sources confirmed the player had not been registered and Madrid admitted they had sent the required documentation after the window closed at midnight local time (2200 GMT).

Madrid had accused United of sending the documents after the Spanish deadline had passed, but the English club laid the blame at Madrid's door and said they were "delighted" by the outcome.

The Madrid club have come in for ridicule in the Spanish press, but Perez, speaking on Cadena Ser radio, denied it was their fault.

"I don't want to pin the blame on anyone, but we cannot understand how it could take eight hours (to return the papers)" he said.

"I don't believe it was done with any bad intention, they just lack experience. The same thing happened over the transfer of (Fabio) Coentrao," he added, referring to a deal to take the Portuguese defender to United which fell through two years ago. "They have a new management team and they are lacking experience."

As a result, de Gea, 24, is forced to remain at Old Trafford, even though he has been in conflict with manager Louis van Gaal over his desire to move.

"He's very upset because he was very keen on the idea of coming here," Perez said.

But he refused to be drawn on whether the club would go back for him in January. "I really have no idea what will happen now,." Perez said. "It's too soon to talk about what the future holds."