The actions of a Chelsea supporter who was photographed making an alleged racist "monkey" gesture during the team's Capital One Cup win over Manchester United on Wednesday are being investigated by police.
The move by Scotland Yard came after the West London club confirmed they had launched their own investigation into the incident. The force said it had not yet made any arrests.
The matter comes at a sensitive time for Chelsea, who have 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 Man 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 serving a four-match ban after being found guilty by the Football Association of racially abusing the Queens Park Rangers defender Anton Ferdinand.