David De Gea's move to Real Madrid has reportedly stalled at the final hurdle as completed paperwork failed to arrive at the Spanish league office before the transfer deadline, according to multiple news outlets.
If the reports are accurate, Manchester United will retain the player at least until January but will miss out on an agreed fee of £29 million.
The deal was also supposed to include Real goalkeeper Keylor Navas coming to Old Trafford, but the LFP office reportedly did not receive the paperwork before the deadline, rendering all the negotiations moot.
Press Association reported the 10-time European champions are still trying to resuscitate the deal, with sources at United insisted they uploaded the requisite documents to FIFA's Transfer Matching System.
Citing sources at Old Trafford, PA also rejected reports in Spain that United submitted paperwork after the deadline. The English club claim they have a time-stamp from FIFA as proof.
De Gea's contract at Old Trafford is due to expire in the summer 2016 and the 24-year-old had rejected lucrative offers of an extension.
United had been willing to hold him to the final year of his contract and risk losing him on a free transfer, but after talks between the two clubs, a transfer was arranged.
United were set to make an £11m profit on De Gea, who they signed in 2011 for £18m. The deal would have been the second-most expensive goalkeeper in football history.
United had originally hoped to secure a record fee for a goalkeeper, topping the £32.6m Juventus paid Parma for Gianluigi Buffon in 2001, but sources said they had compromised by the end.
The dramatic 11th-hour collapse came as a huge shock to determined Madrid, and De Gea himself was eager to complete his return to Madrid this summer.
The 24-year-old, who joined United from Real's neighbours Atletico Madrid four years ago, hails from the Spanish capital and his pop star girlfriend Edurne Garcia also lives in the city.
Manager Louis van Gaal has omitted De Gea from the United squad for their first six matches of the season amid concerns about his state of mind.
De Gea has made 175 appearances and won the club's Player of the Year award for each of the last two seasons.
De Gea's sale would have taken United's summer income from transfers to around £100m with Angel Di Maria joining Paris Saint-Germain for £44m and Javier Hernandez, Rafael da Silva, Jonny Evans, Robin van Persie and Nani also sold.
United are also closing in on the signing of 19-year-old French forward Anthony Martial from Monaco for £36m.
Costa Rica international Navas, 28, joined Real from Levante last year for €10m but only made 13 appearances for the 10-time European champions, after starring at the World Cup in 2014.