UK police are to examine video footage in a bid to identify Chelsea football fans who appeared to stop a black man from boarding a Paris train.
The amateur video shows a man being repeatedly pushed from the train while a group chants, "we're racist, we're racist and that's the way we like it".
Scotland Yard said it was taking the incident "very seriously".
Chelsea condemned the incident and issued an appeal for witnesses, saying any fans involved faced a ban.
The incident was filmed by a British man ahead of Tuesday night's Champions League match, which the London club drew 1-1 with Paris St Germain.
The Metropolitan Police said it would help French authorities investigate and would support them in any action they chose to take.
"We will examine the footage with a view to seeing if we can apply for football banning orders, preventing people from travelling for future matches," a spokesman said.
The footage was obtained by the Guardian, which reported that the incident had happened at Richelieu-Drouot station in the centre of the French capital on Tuesday evening.
British expatriate Paul Nolan, who filmed the incident on his phone, told the BBC it had been an "ugly scene" and "very aggressive".
In a statement, Chelsea condemned the behaviour as "abhorrent" and said the fans' actions had "no place in football or society".
The Premier League leaders vowed to support any criminal action and appealed for witnesses to contact the club.
"Should evidence point to the involvement of Chelsea season ticket holders or members of the club, we will take the strongest possible action against them," the statement said.
This could include banning them from future matches, it added.
Prime Minister David Cameron told LBC Radio the footage looked "extremely disturbing and very worrying".
"It's obviously potentially a criminal offence and so I'm sure the French police will be looking at it very seriously."
The abuse has drawn widespread condemnation from the world of football.
Uefa, which runs the Champions League, said it was "appalled" by the incident but it would not be investigating.
"As it occurred away from the stadium, it is outside Uefa's remit to act," it said in a statement.
English football's governing body, the Football Association, said it "fully supports Chelsea's position in seeking to ban any of the club's season-ticket holders or members who face criminal action in relation to these abhorrent scenes".
Backing for Chelsea
Sepp Blatter, president of world football's governing body Fifa, tweeted that there was "no place for racism in football".
Lord Ouseley, chairman of Kick It Out, which campaigns against racism in football, said the fact the incident involved an assault on the man was "even more shocking".
The Football Supporters' Federation (FSF) said the overwhelming majority of Chelsea fans would be "disgusted" by the incident.
"There's no place in society for this type of behaviour, and we back Chelsea FC's strong stance," the FSF said.
Paul Canoville - the first black footballer to play for Chelsea - told the BBC he was saddened by what had happened.
"For me as a black player, and other black players, it would hurt, most definitely.
"It is haunting. It wasn't nice seeing it, hearing it, at all," he said.
Frank Sinclair, a black footballer who played for Chelsea more than 150 times, said the men in the video had nothing to do with his former club.
"They tend to move from club to club, they drift and they look at an opportunity where they might have got tickets on the black market, decided to go to this game to cause problems," he said.
"Certainly, they're not represented by Chelsea Football Club."
Former England captain and BBC football pundit Gary Lineker tweeted: "Just saw the video of the Chelsea mob's deplorable behaviour and racist chants.
"Sickening, embarrassing, moronic, disgraceful."