Ryanair is facing prosecution and a substantial fine for repeatedly misleading passengers about the availability of its cheapest fares.

The airline has breached the Advertising Code seven times in the past two years and has failed to heed warnings by the Advertising Standards Authority.

The authority is publishing today its decision to refer Ryanair to the Office of Fair Trading, which has the power to take advertisers to court, where they can be fined or made subject to injunctions prohibiting them from repeating false claims. The authority said that it had no choice but to refer Ryanair to the OFT because the airline was refusing to cooperate and failing to provide evidence to support the claims that it was making.

Ryanair had advertised prices that did not include taxes and charges and had failed to make clear “significant restrictions that would exclude customers from taking advantage of an offer”, the authority found. The airline had also made misleading and denigratory comparisons with competitors.

Christopher Graham, the director-general of the authority, said: “It is very disappointing, but absolutely necessary, that we have had to take this course of action. The ASA has given Ryanair every opportunity to put its house in order.”

In January Ryanair refused to withdraw an advert of a woman dressed as a schoolgirl, despite the authority’s ruling that it breached advertising rules on social responsibility and decency. Ryanair accused the authority of censorship, saying that it was run by “unelected, self-appointed dimwits”.

The authority also upheld a complaint that Ryanair had published misleading information about the effect of aviation on climate change. A Ryanair spokesman said that ASA stood for “Absolutely Stupid Asses”.

Ryanair lost £20 million in February after closing its website to comply with an OFT ruling that it should advertise prices inclusive of taxes and charges.

The airline said after the latest ASA announcement that it would ask the OFT to investigate the authority’s “unfair procedures, bias and false judgments”.

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