A police investigation is under way after a Chelsea supporter made an alleged racist gesture towards a Manchester United player.
A picture published in the national press appeared to show the fan making a "monkey" action during the Blues' Capital One Cup fourth-round win over Manchester United at Stamford Bridge on Wednesday night.
Chelsea have also launched their own investigation into the matter and promised full assistance to the police.
A statement from the Metropolitan Police read: "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."
The incident comes at a sensitive time for Chelsea having lodged a formal complaint with the Football Association against referee Mark Clattenburg for using "inappropriate language" towards midfielder John Obi Mikel.
That matter is also being investigated by police. Clattenburg maintains his innocence.
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."
Chelsea have been involved in a series of race-related issues over the past year.
In March the club banned for life a supporter who admitted racially abusing former player Didier Drogba.
The most high-profile incident concerned captain John Terry, who is currently serving a four-match domestic ban after being found guilty of racially abusing QPR defender Anton Ferdinand by the FA.
Press Association Sport also understands the club are looking into a number of incidents concerning fans in the United section. One supporter did get on to the field while objects were also thrown on to the playing surface.
Chelsea won the match 5-4 after extra time.end