Claims that a group of Chelsea football fans were involved in racist chanting on a train back from the club's 0-0 draw against Norwich City 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 website said: "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."