A Transport for London (TfL) ad featuring a black teenage boy verbally harassing a white girl has been banned for “perpetuating the negative racial stereotype about black men as perpetrators of threatening behaviour”.

The Advertising Standards Authority (ASA) said the “irresponsible” ad – which was the subject of a complaint – featured a “harmful stereotype”.

TfL said the ad was one of three that ran on Facebook, each a cut down section of a two-minute film, called Would you know how to act like a friend?, part of a campaign to encourage Londoners to intervene safely if they witness sexual harassment or hate crime on the public transport network.

The company, which runs the capital’s transport network including the underground and bus system, said the other two shortened ads showed a white man committing a hate crime against a black woman, and a white man committing a hate crime against another white man.

“The [overall] film featured a diverse cast,” TfL told the ASA in its response to the complaint.

TfL also said the fuller story, in the two-minute film, showed two male youths perpetrating an incident of sexual harassment in which both characters “intimidated the victim and displayed offensive behaviour”.

While the main perpetrator who verbally harassed the young girl was a black youth, he was accompanied by a white male friend who sat close to the victim, “boxing her in”, said TfL.

TfL said that, according to its statistics, the probability of a typical target Facebook user seeing only one of the three ads was 10%, and there was only a 2% chance of a user only seeing the cut-down version that was the subject of the complaint.

However, the ASA said it was still possible to see the ad in isolation, and that it could still cause harm or offence, and assessed the ad as it would have appeared in a users’ feed on Facebook.

“We understood there was a negative racial stereotype based on the association between black males, including teenagers, and threatening behaviour,” said the ASA. “We assessed whether the ad reinforced that stereotype.”

The ASA disagreed that the ad showed the white and the black male characters displaying intimidating and offensive behaviour.

“While the white male friend was shown in the ad and the two-minute film, the ad did not show him as jointly intimidating the victim,” said the ASA. “The only aggressor in the ad was the black teenage boy.”

The ASA said that, when seen in isolation, the ad had the effect of “perpetuating a negative racial stereotype about black men as perpetrators of threatening behaviour”.

“We concluded that the ad featured a harmful stereotype, was irresponsible and likely to cause serious offence,” said the ASA, which banned it from running again. “We told TfL to ensure that future ads were socially responsible. We also told them to avoid perpetuating harmful stereotypes and causing serious offence on the grounds of race.”

The TfL campaign launched last October, during national hate crime awareness week.