The Metropolitan Police have confirmed they have opened an investigation into alleged racist behaviour by a Chelsea supporter.
A photograph published in the national press appeared to show a fan making a 'monkey' gesture during the Blues' Capital One Cup fourth-round win over Manchester United at Stamford Bridge.
The move by police comes after the club confirmed they had launched their own investigation into the incident.
A statement from police read: "Today, Thursday, 1 November, police have received a complaint regarding alleged racist behaviour at Stamford Bridge last night, Wednesday 31 October.
"Chelsea FC is committed to removing all forms of discrimination" - Chelsea statement
"An investigation has been launched. There have been no arrests and enquiries continue."
The matter 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.
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 ban after being found guilty of racially abusing QPR defender Anton Ferdinand by the Football Association.
Press Association Sport understands Chelsea were planning to contact police themselves regarding this latest matter.
The club are also understood to be looking into a number of incidents concerning United fans after a pitch invasion and some objects were thrown onto the pitch during the match, which Chelsea won 5-4 after extra time.