The ad watchdog had formally investigated complaints about Ryanair's ads and found them in breach on nine occasions over a two-year period.
This includes a ruling today that find Ryanair in breach of the advertising code once again for failing to ensure flights were available in sufficient quantities following a press ad claiming it was offering 2m seats from £10 one way.
The ASA said: "Through misleadingness and unfair comparisons Ryanair has repeatedly breached the CAP Non-broadcast Advertising Code, which is designed to protect consumers and promote fair competition."
The ASA found that Ryanair was persistently misleading consumers by making "exaggerated claims" about the extent of availability of flights at the advertised price and advertising prices that were not inclusive of taxes and charges
Other breaches included making "misleading and denigratory" comparisons with competitors, not stating clearly significant restrictions that would exclude customers from taking advantage of an offer, and failing to provide evidence to prove the claims they were making.
Christopher Graham, director general of the ASA, 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 and ensure that its advertising adheres to the codes.
"Instead, they have continued to mislead consumers and denigrate competitors. We would prefer to work with advertisers within the self-regulatory system rather than call in a statutory body, but Ryanair's approach has left us with no option but to refer them to the OFT who will consider appropriate action."
The OFT acts as the legal backstop to the ASA, but it is rare for the watchdog to make a referral to the OFT, the most recent case being in 2005.Last Friday, in a preemptive strike at the ASA, Ryanair announced it was making a complaint to the OFT about the watchdog after learning that it was referring the airline to the OFT this week.