UEFA, European football's governing body, has confirmed it is looking into a complaint of racist abuse by Benfica supporters during United's recent Champions League match.
The Reds have confirmed they made a complaint after the match at Estadio da Luz on 26 September.
At this time, United are making no further comment on the alleged incidents as UEFA carry out their investigative procedures.
Europe's governing body released a statement announcing their intentions.
"[We] are investigating a written complaint made by Manchester United about alleged racist conduct by spectators at the UEFA Champions League Group F match against Benfica," read the statement.
"The English club has submitted the complaint, following allegations by United players that they were subject to abuse of a racist nature at the game in Portugal, which United won 1-0."