West Ham's move for Noussair Mazraoui has reportedly collapsed, leaving the door pen for Man United to swoop in.
The news broke on Thursday that the Hammers had agreed a £13.5m fee with Bayern Munich for the right back plus £3m in add-ons.
However, the deal has fallen apart as the two sides were unable to agree on terms.
Sky reporter Florian Plettenberg tweeted: 'Noussair Mazraoui deal with West Ham is OFF! no agreement on agent‘s side has been reached.
'Despite the agreement between the clubs and on player‘s side West Ham now have left the negotiations.'
He then added: 'Manchester United now pushing to sign Mazraoui. Advanced talks.'
Sky also report that the future of Aaron Wan-Bissaka will be 'key' to whether or not a move for Mazraoui goes through.
The English right-back, who is in the final year of his contract at Old Trafford, has been linked with West Ham and Galatasaray and would raise valuable funds for the club.
Mazraoui, 26, has spent two seasons at the Allianz Arena, having joined on a free transfer from Ajax in the summer of 2022.
The full-back came through the ranks at Ajax, where he played under current Man United boss Erik ten Hag.
He was also a member of the Morocco team that overcame the odds to reach the semi-final of the 2022 World Cup.
West Ham, now managed by Julen Lopetegui, have made three major signings so far in this transfer window.
Exciting youngster Luis Guilherme joined the club from Palmeiras, while goalkeeper Wes Foderingham arrived on a free transfer following his departure from Sheffield United.