The Portuguese has endured a difficult time of late at Old Trafford with United slipping to eighth place in the league, already 18 points behind leaders and rivals Manchester City after 15 games.
Mourinho has often had to defend his record in the wake of mounting criticism. There has been speculation that the club may look to replace him in the near future, despite signing a contract extension in January that ties him to the club until 2020.
However, numerous British media outlets quoted a rare statement from Mendes on Friday rejecting any suggestion that Mourinho was close to leaving.
"There have been more rumours of Jose Mourinho leaving Manchester United. It's totally untrue," Mendes was quoted as saying.
"Jose is very happy at the club and the club is very happy with him. He has a long-term contract with Manchester United and is fully committed to the club in building a solid winning project."
United are without a win in four matches in the league. They host bottom-of-the-table Fulham on Saturday.
They have also reached the last 16 of the Champions League ahead of their final group stage game at Valencia on Wednesday. (Reporting by Christian Radnedge; Editing by Hugh Lawson)
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