Anti-discrimination group Kick It Out has contacted Manchester United to ask them to stop the club’s fans singing a “racist” chant about Romelu Lukaku.
It says the lyrics, about the size of the Belgium striker’s penis, are “offensive and discriminatory”.
“Racist stereotypes are never acceptable, irrespective of any intention to show support for a player,” said Kick It Out.
A United blog has already asked the club’s fans to refrain from the song.
“It’s a cheap and insulting stereotype that has no place in this day and age,” United fan Scott Patterson wrote in a post on The Republik of Mancunia.
Lukaku joined United for an initial £75m from Everton during the summer in a deal that could rise to £90m.
Kick It Out said it was “aware of footage of alleged racist chanting by supporters of Manchester United that emerged on Wednesday 13 September”, Lukaku having scored in the game against Basel in the Champions League the previous evening.
“We have contacted Manchester United regarding the issue and will be working closely with them and the Football Association to ensure that it is addressed swiftly,” said a Kick It Out spokesperson.
“If we receive any reports relating to the discriminatory chant, those will be passed on to the governing body and the perpetrators can expect to face punishment.”
BBC Sport has contacted Manchester United for a comment on the issue.