Claims that a group of Chelsea fans were involved in racist chanting on a train back from the club's 0-0 draw at Norwich are being investigated.
British Transport Police said they had been made aware of the alleged incident after the clubs' Premier League match on Saturday.
"We were made aware of the incident on board the 1600 Norwich to London Liverpool Street Service,'' a spokeswoman said. "It is alleged that some individuals were involved in racist chanting.
"British Transport police has contacted the individual concerned who made the allegations and an investigation is ongoing.''
Chelsea has vowed to take action against any fans proved to be involved in racist chanting.
A statement on the club's official website read: "Chelsea Football Club is concerned to hear that a small number of fans travelling back by train from Norwich were alleged to be involved in racist chants and comment.
"We are working to help the relevant authorities identify those responsible and will take the strongest possible action should these allegations prove to be true.
"The club, like the overwhelming majority of our fans, strongly believe that all forms of discrimination are abhorrent and have absolutely no place in society.''
The alleged incident comes just a week before Chelsea face QPR in a potentially explosive FA Cup tie at Loftus Road.
Captain John Terry has been charged with racially abusing QPR defender Anton Ferdinand during the Premier League game between the two clubs on October 23.
Terry, who denies the allegations, is due to appear at West London magistrates court on February 1.
Chelsea were forced to condemn supporters who were clearly heard chanting "Anton Ferdinand, you know what you are'' in their 1-1 Champions League Group E draw at Genk's Cristal Arena on November 1.
Manager Andre Villas-Boas does not want any such issues when they return to Loftus Road next Saturday, in a match which will kick off at noon - partly because of police advice.
''Regarding the fans, it is our responsibility as a club to calm the situation around the game and also to put on a message for the media as you can do excellent work in calming things down and everyone can have an impact for just another game of football,'' said the Chelsea boss, who has no intention of leaving Terry out of the squad next weekend.