Chelsea and the Metropolitan Police are investigating after an incident of alleged racist abuse during their Premier League match with Manchester City was highlighted on social media.
Twitter users highlighted a first-half incident when a supporter appeared to abuse City forward Raheem Sterling when the England international retrieved the ball near the sideline at Stamford Bridge.
"We're aware of reports and video footage. We will investigate the matter and take the strongest possible action where necessary," a Chelsea spokesman said.
No arrests have been made but The Met Police are investigating the claims.
"We are aware of a video circulating online in which it is claimed racial abuse was allegedly directed at a player at a Chelsea v Manchester City game at Stamford Bridge on Saturday, December 8," a police statement said.
"We will review the footage to determine whether any offences have been committed."
Chelsea are seeking to establish the facts of the incident.
Should any offence have been made, a life ban could be handed to the supporter or supporters responsible.
Chelsea won the match 2-0 as the champions lost for the first time this season.