They are claiming that the English side sent the documents needed to complete the deal after the Spanish transfer deadline had passed.

Madrid seemed set to conclude the long-running transfer saga with a deal that would see the Spanish giants pay a significant transfer fee for De Gea and send Costa Rican international goalkeeper Keylor Navas to United as part of the deal.

However, Spanish league sources confirmed that the player hadn't been registered and Madrid admitted they had sent the required documentation to the league after the window closed at midnight local time (2200GMT) on Monday.

"Manchester United introduced into the Transfer Matching System (TMS) the details for the transfer of David de Gea, not those for Keylor Navas, at 00:00 Spanish time, sending simultaneously to Real Madrid the signed transfer contracts. Real Madrid received the complete documentation at 00:02 and tried to access the TMS but this was already closed," Madrid said in a lengthy statement posted on their website on Tuesday.

"Madrid, facing the possibility of a contentious situation in the transfer of the player, decided to send the contracts to the Spanish Professional League, even knowing the deadline had already passed.

"In conclusion, Real Madrid did everything necessary in every moment to try to finalise these two transfers."

Despite Madrid's prolonged interest in De Gea, they claim negotiations between the two clubs for the player didn't begin until Monday morning.

Real Madrid haven't indicated whether they intend to appeal to football's governing body FIFA in an attempt to force the transfer through.

FIFA refused to comment on the specific details of the case when contacted by AFP on Tuesday, but confirmed that no appeal had been lodged by any of the parties involved.

"Up until now, we do not appear to have been contacted by either of the parties in this matter," said a FIFA spokesperson.

De Gea has just one year left on his contract at Old Trafford meaning he could join Madrid for free next year if the transfer doesn't go through now.

The Spanish press slammed Madrid's failure to deliver their prized target in the transfer market this summer with the front pages of sports newspapers Marca and Sport describing the situation as "ridiculous."

The Spanish international hasn't featured in any of United's six games so far this season with manager Louis van Gaal claiming the 24-year-old wasn't focused at the English giants due to Madrid's interest.

Madrid were in the market for a goalkeeper after Spain captain Iker Casillas ended his 25-year association with the club by moving to Porto in July.

De Gea joined United from Real's city rivals Atletico Madrid in 2011 and, despite a shaky start went on to shine, winning the club's fans' and players' player of the year awards for the past two seasons.

However, he refused to sign a new deal with United due to his desire to return to Spain with Real.

A standoff between the two clubs ensued with United also expressing their interest in Real captain Sergio Ramos over the summer before he ended speculation over his future by signing a new five-year deal with Los Blancos.