The Committee of Advertising Practice has issued an advertising alert via an email to all its members, especially media owners, with a copy of the Ryanair ad and instructions not to accept it if it is presented.
The ad, which has already appeared in the Herald, the Daily Mail and the Scottish Mail, received 13 complaints from members of the public who found the 'Britney Spears' style creative offensive and said that it carried sexual connotations.
Under the heading "Hottest back to school fares", the press ad features a picture of a teenage girl standing in a classroom, wearing a version of a school uniform consisting of a short tartan skirt, a cropped short sleeved shirt and tie, and long white socks.
The ASA instructed Ryanair to withdraw the ad because it considered it to be irresponsible and likely to cause widespread offence.
Peter Sherrard, Ryanair head of communications, said: "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.
"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."
Sherrard said Ryanair believed there was nothing irresponsible or offensive in the ad.
He said: "Consequently we will not be withdrawing this ad and we will not provide the ASA with any of the undertakings they seek."