Budget airline Ryanair has been told to withdraw an advert featuring a model in schoolgirl-style clothes and a headline "hottest back to school fares".
A model posed in a classroom in the controversial Ryanair advert
The Advertising Standards Authority (ASA) said the "irresponsible" image appeared to link teenage girls with sexually provocative behaviour.
The advert shows the model with a bare midriff in a short skirt, tie, shirt and knee-high socks in a classroom.
Ryanair refused to withdraw the image, but ASA dismissed this as "PR bluster".
The advert has been removed and it will not run again, an ASA spokesman told BBC News.
Media space owners will not accept the advert again, the spokesman said.
Ryanair, in turn, described the ASA ruling as "censorship".
The advert, which was printed in three newspapers reaching 3.5 million readers - the Herald, Daily Mail and Scottish Daily Mail - prompted 13 complaints to the ASA from readers who found it offensive.
After an investigation, the watchdog ruled the advert breached the advertising code's rules on social responsibility and decency.
"We considered that her appearance and pose, in conjunction with the heading 'Hottest', appeared to link teenage girls with sexually provocative behaviour and was irresponsible and likely to cause serious or widespread offence," the watchdog said.
The ASA told Ryanair to withdraw the advert and ensure its future promotions complied with the advertising code.
However, the airline said the model's clothing reflected what was currently fashionable among young women and that the number of complaints was insignificant compared to the three newspapers' combined readership.
Ryanair also said its advert was considerably less suggestive than some others appearing in the media.
"It is remarkable that a picture of a fully-clothed model is now claimed to cause 'serious or widespread offence', when many of the UK's leading daily newspapers regularly run pictures of topless or partially-dressed females without causing any serious or widespread offence," said Peter Sherrard, head of communications for the airline.
"This isn't advertising regulation, it is simply censorship. This bunch of unelected self-appointed dimwits are clearly incapable of fairly and impartially ruling on advertising."
Mr Sherrard added that Ryanair believed the advert was not irresponsible nor offensive and would therefore "not be withdrawing this ad" and would "not provide the ASA with any of the undertakings they seek".
Ryanair's stance was dismissed by the ASA spokesman as "public posturing".
The Herald, which received a complaint from a reader, said they were not prepared to run it again.
The Daily Mail and Scottish Daily Mail had not received any complaints from readers but said they would not run the promotion again.