London's Metropolitan Police Force announced on Thursday that they had launched an investigation after a Chelsea supporter was photographed apparently making a racist gesture towards a black Manchester United player.
The picture, published in several British newspapers, was taken during Chelsea's dramatic 5-4 League Cup win over Manchester United at Stamford Bridge on Wednesday and appeared to show a fan making a monkey gesture at striker Danny Welbeck.
"We have received a complaint into alleged racist behaviour at Stamford Bridge on Wednesday," a police spokesman told AFP on Thursday. "No arrests have been made and enquiries are continuing."
The police's move came after Chelsea had announced earlier Thursday that they were holding their own separate investigation into the incident and appealed for anyone who could identify the fan 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 club added in a statement.
The incident comes after Chelsea confirmed they had complained to the Football Association about "inappropriate language" used by referee Mark Clattenburg towards midfielder Jon Mikel Obi during Sunday's Premier League defeat by United.
A similar complaint involving Spanish midfielder Juan Mata was dropped by the club.
However, police are investigating Clattenburg's alleged comments to Mikel.
Chelsea captain John Terry is currently serving a four-match ban imposed by the FA for racially abusing Queens Park Rangers' Anton Ferdinand during a match in October last year.
Earlier this year Chelsea banned for life a supporter who admitted racially abusing former player Didier Drogba.