Police are investigating chants aimed in the direction of Leicester City supporters by their East Midlands rivals Nottingham Forest.
They are treating the chants as a potential hate crime.
Nottinghamshire Police said it was investigating the "racist" chanting after a video-clip filmed during the club's FA Cup tie against Leicester was posted online.
The footage, thought to be from the early stages of last Saturday's 0-0 draw at The City Ground, shows Forest fans singing: "You used to be English, you're not any more."
In a statement released by Nottinghamshire Police, Superintendent Mark Holland said: "The incident is being treated as a hate crime and we are reviewing CCTV footage and following up on a number of lines of inquiry.
"We take all reports of racism very seriously and work extremely closely with the football clubs in Nottinghamshire in a bid to kick racism out of the game."
Commenting on the police inquiry on Twitter, some fans claimed the chant was a reference to Leicester's high ethnic minority population, while others suggested that it alluded to the Thai-led consortium which owns Leicester City.
A total of 11 arrests were made in connection with Saturday's FA Cup tie, none of which related to racist behaviour.
A Nottingham Forest spokesman said the club had a zero tolerance attitude towards any inappropriate language at The City Ground.
"Any incidents of racial or abusive language reported to us during the game against Leicester City were dealt with immediately and appropriate action was taken," the spokesman said.
"The club are continuing to assist the police with their investigation into this matter".