The four-minute video shows an increasingly ill-tempered exchange in which a woman – apparently of European origin – provokes outrage among astonished passengers when she tells a black man: “You have a problem because you guys used to be slaves”.

It is not clear when the footage was filmed, but it is believed that the row erupted on a Jubilee line train between Stratford and West Ham. The man can be heard shouting at the woman, claiming she told him: “Don’t touch me” after he accidentally bumped into her.

The woman says: “You called me racist, I’m not racist. That is why people have a problem with you guys. I have black friends. I’m not racist.”

As the male passenger continues to shout, she adds: “It’s not about the skin. You can be pink. If you acted like a human being I wouldn’t have a problem with that. You have a problem because you guys used to be slaves.”

At this point other passengers shout at her to “shut up” and “don’t say that”. One says: “You can’t say that – this is England.”

The man then gets up from his seat and repeatedly asks the woman: “Did you just call me a slave?”

The woman eventually says: “You’re a piece of s**t. Honestly, I’m fed up with you guys” before she moves across the carriage and out of shot.

British Transport Police launched an appeal for witnesses on Friday, and now say that a person has come forward. “A woman, from Essex, will be interviewed in due course,” a spokesman told the London Evening Standard. “Officers are still appealing for the person who recorded the video, and all those involved in the altercation, to come forward.”

See the video, posted on YouTube by Hels_Montana, here: