A Ryanair advert showing what appears to be a skimpily clad schoolgirl in a classroom and promising "hottest back to school fares" was irresponsible and offensive, Britain's Advertising Standards Agency (ASA) ruled.

Upholding complaints from 13 newspaper readers, the agency said on Wednesday that the advert's classroom setting and the model's outfit -- wearing knee-length white socks and a tie and clutching books -- strongly suggested she was a schoolgirl.

"We considered that her appearance and pose, in conjunction with the heading 'hottest', appeared to link teenage girls with sexually provocative behavior," the agency said.

The advert, showing the model in a short tartan skirt and cropped shirt that exposed her midriff, was therefore "irresponsible and likely to cause serious or widespread offence", the agency concluded.

Ryanair, well known for its publicity stunts, accused the ASA of "subjective censorship", saying its fully-clothed model contrasted with pictures of topless women or celebrities in various stages of undress that often appear in British papers.

Europe's biggest low-cost carrier did not address the agency's central complaint, however, that the setting of the advert implied the model was portraying a schoolgirl.

"We will not be withdrawing this ad and we will not provide the ASA -- Absurd Silly Asses -- with any of the undertakings they seek," Ryanair spokesman Peter Sherrard said in a statement.

The ASA, however, said the three newspapers who originally ran the advert -- the Herald, Daily Mail and Scottish Daily Mail -- had all pledged not to use it again.

Ryanair apologized on Monday after French President Nicolas Sarkozy threatened legal action over another advert depicting him and girlfriend Carla Bruni daydreaming about a wedding.

The airline was less repentant earlier this month when it labeled a Seville-based consumer group "Spanish Prudes" for complaining about a 2008 calendar showing Ryanair hostesses in bikinis.

Previous Ryanair adverts have lampooned rivals as well as British Prime Minister Gordon Brown and Irish counterpart Bertie Ahern. Chief Executive Michael O'Leary once dressed as the Pope to preach a "low-fares sermon", earning him a rebuke from the Vatican.