Police have started an investigation after a fan made what appeared to be a racist gesture in Chelsea's League Cup win over Manchester United.
Pictures appear to show the man making a 'monkey' gesture during Wednesday's fourth-round tie at Stamford Bridge.
United striker Danny Welbeck appeared to be the target of the alleged abuse after losing possession.
Chelsea had already started their own investigation into the incident, saying they would examine the footage.
"Police have received a complaint regarding alleged racist behaviour at Stamford Bridge on Wednesday," read a Metropolitan Police statement.
"An investigation has been launched. There have been no arrests and enquiries continue."
A Chelsea spokesman said: "If we have sufficient evidence we will take the strongest possible action, including supporting criminal prosecution."
The incident comes after the Blues accused referee Mark Clattenburg of using "inappropriate language" towards midfielder John Mikel Obi in Sunday's Premier League match against United. The west London club has made a formal complaint to the Football Association against Clattenburg.
Earlier this year, Chelsea imposed a lifetime ban on a supporter who admitted racially abusing former Chelsea striker Didier Drogba.