Unlike ads that appear in other forums, like television, online and print, he said, passengers on public transportation “cannot simply switch off or turn a page if an advertisement offends or upsets them — and we have a duty to ensure the copy we carry reflects that unique environment.”

Image Sadiq Khan, the mayor of London, wants to ban from the city’s public transportation system ads that promote unhealthy or unrealistic body images. Credit... Yui Mok/Press Association, via Associated Press

The announcement of the ban inspired mixed reactions, with some saying that Mr. Khan, a member of the Labour Party, had performed a public service by fighting the regular portrayal of unrealistic body types.

“A great start for London’s ‘feminist Mayor’ ” Women’s Equality Party wrote on Twitter.

But some critics suggested that Mr. Khan, a Muslim, was using the ad ban to clamp down on racy images of women because they were counter to the norms of conservative Islam. Mr. Khan’s office declined to comment when asked about that line of criticism.

Mr. Khan is the son of a bus driver who grew up in public housing, and his religion drew attention in the mayoral race, with a rival candidate, Zac Goldsmith, accusing him of giving “oxygen and cover” to extremists.

Ian Twinn, a spokesman for the Incorporated Society of British Advertisers, which represents about 450 British companies, said he had no problem with the regulation as long as it focused solely on not promoting unhealthy body images.

“Parents don’t want their children to see ads of models who are unhealthily thin children, and send out a message that this is what you should look like,” he said. “Equally, we don’t want people who are comfortably chubby threatening to ban perfectly reasonable ads. We are not meant to go around with excessive fat on our waists. If fat is the new normal, we don’t want that to be the reason to censor people who are not fat. That seems barmy.”

Mr. Twinn said the model in the Protein World ad had a toned and athletic body, a perspective that was echoed by the Advertising Standards Authority.