Both the Metropolitan Police and Chelsea are investigating an apparent monkey gesture made by a supporter during Wednesday's Capital One Cup tie against Manchester United.
The Blues have launched an investigation into the matter, while the Met Police have confirmed they will also probe alleged racist behaviour by a supporter.
A Met police statement declared: "Today, Thursday, November 1, police have received a complaint regarding alleged racist behaviour at Stamford Bridge last night, Wednesday, October 31. An investigation has been launched. There have been no arrests and enquiries continue."
While a statement from the club read: "Chelsea Football Club can confirm we are investigating an incident involving a member of the crowd at last night's game against Manchester United. The club will be examining all available footage and asks anyone who can identify the individual to contact the club.
"Chelsea FC is committed to removing all forms of discrimination and if we have sufficient evidence we will take the strongest possible action, including supporting criminal prosecution."
The issue follows controversy at the weekend when referee Mark Clattenburg was accused of inappropriate language directed at Chelsea's John Obi Mikel.
Chelsea lodged a formal complaint to the Football Association on Wednesday and, although there was no mention of what Clattenburg is supposed to have said, it is understood the basis of their complaint is that the official used language of a racist nature.
Information from the Press Association was used in this report