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 24-year-old 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.
Manchester United say they will liaise with Kick it Out on the matter.
The Old Trafford club are also waiting to learn whether the chant about the Belgium forward is regarded as racist for criminal purposes and is therefore actionable.
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."
'It is a chant that is racist'
Clifford Stott, a professor of social psychology at Keele University, is an expert on crowd behaviour.
"Football fans have always chanted in a way that is sometimes on the edge," he told BBC Radio 5 live. "This is another example of it. I suspect that it will rapidly die away.
"I think a lot of fans who sang it had no racist intent and don't see it as racist - they see it as funny.
"It is a chant that is racist and Manchester United fans need to face up to that fact.
"It does perpetuate a racist stereotype and I don't think it's a chant that is going to survive for any great length of time because people are realising that fact."