Manchester United on Sunday at Old Trafford ejected a fan from the game with Liverpool for allegedly racially abusing Trent Alexander-Arnold.
Security ejected the supporter after being alerted that he had allegedly hauled insults at the Liverpool defender.
The incident happened during the first half of the match close to the players’ executive boxes in the Stretford End.
Supporters who witnessed the incident are reported to have immediately called for stewards to remove the fan.
The Old Trafford club says the fan will be banned for life if it is proved an offence was committed at Sunday’s game.
A United spokesman said the matter was being investigated “as a priority”.
A club statement added: “Racism and all forms of discrimination are completely unacceptable and do not reflect the values of our club.”
“We are aware of an incident at yesterday’s game where an individual in the home section has allegedly engaged in racial abuse.
“The individual was identified and ejected from the stadium.”
Liverpool welcomed the “swiftness with which Manchester United have launched an investigation” and said the club would “support the strongest possible action being taken against anyone found to be responsible”.
United drew 1-1 with Liverpool to end the Premier League leaders’ 100% record.
Marcus Rashford gave United the lead from close range in the 36th minute as Liverpool’s cries for a foul on Divock Origi in the build-up were denied by VAR.
Then VAR correctly ruled out what would have been a Sadio Mane equaliser in the 44th minute for a handball.
Both decisions prompted wild celebrations from the home crowd, but Liverpool, who were looking for a record-equalling 18 straight Premier League wins, levelled late on through substitute Lallana’s back-post tap in.
The result moves United into 13th, seven points off the top four, while Liverpool sees their lead at the top cut to six points following Manchester City’s 2-0 win at Crystal Palace on Sunday.