The chief superintendent of the Metropolitan Police's Public Order branch, Mick Johnson, says his force is working closely with football clubs at tackling racism in the light of further allegations which surfaced on Monday night.
A television report on the matter appeared to show Millwall fans persistently subjecting Leeds striker El-Hadji Diouf to racial abuse when his side played at the Den on November 18. Sky Sports News footage appeared to show the Senegal international alerting the referee, Mark Halsey, and making stewards aware of his allegations.
Millwall released a statement in which they said they were "appalled" by the footage they had witnessed. Chief executive Andy Ambler said: "Clearly that behaviour is unacceptable in any football ground in the country and at Millwall we will ban the individuals for life and we hope that Sky will pass the footage to the police so that further action can be taken."
Johnson said: "It is important that we work with football clubs and the football authorities, including the FA, to ensure that there is no misunderstanding about what constitutes racist language or behaviour and that we work with clubs to educate, encourage and empower supporters to stamp out racism in football.
"Some words which historically have been perceived by some as acceptable cause harassment, alarm or distress to others, and people who use this language could be committing a criminal offence.
"Offensive language, within football chants or otherwise, has no place within football, or indeed in society, and those who engage in such behaviour should be under no illusion that they are committing a crime.
"If it is unacceptable outside football then it is unacceptable inside football. We are working closely with partner agencies and all football clubs to get this message across to their supporters."
Bolton forward Marvin Sordell has previously made allegations of racial abuse towards Millwall's fans. In the wake of him doing so, a banner bearing the words "Sordell is a c***" was unfurled at a later game, while a 13-year-old boy was banned from the ground.
Sordell, who represented Great Britain at the London 2012 Olympics, has faced racial abuse on social media websites and claims he is not shocked that such things still occur.
He said: "We're a developed, multi-cultural society. It's surprising it can still go on - but it doesn't exactly shock me. We can't be silent about it. We need to make people aware that there is a problem going on and only when people are aware of the problem can it be sorted out."