Prime Minister David Cameron says the racially-motivated incident involving Chelsea fans in Paris on Tuesday is "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," he told LBC radio.
"I know the British police will give every assistance that they can. I'm sure Chelsea will co-operate with that fully. These are very, very serious matters."
Chelsea have issued a plea to fans to keep their support 'positive' in Sunday's Capital One Cup final against Tottenham
Chelsea have issued an apology to the black man who was racially abused by club supporters on the Paris Metro on Tuesday night
Jose Mourinho and Chelsea feel "ashamed" about the racist incident involving fans in Paris, but he refuses to be connected with culprits