The advertising watchdog received a single complaint from a reader who said that he believed it was misleading because he was unable to book a flight to Malaga for the advertised price of £39.99.
The ASA also challenged Flybe's failure to make sufficiently clear that the advertised prices were "from" prices and the failure to include the compulsory credit or debit card fee in the headline prices.
Flybe supplied evidence to show that at least 10% of the overall number of seats available to Malaga from August 21 2007 to March 29 2008, the period covered by the offer, were available at £39.99.
The company also said that "fares from" was normally included in their advertising but was omitted in error.
Flybe said that because customers were able to pay by Electron without incurring a charge, the handling fee for using credit or debit cards was an additional charge and not a compulsory charge applied to all fares.
The ASA acknowledged that the ad warned the fare was subject to availability but considered that this was not sufficient to warn consumers that the offer was severely limited during some of the travel period.
It concluded that because 10% of seats were not available at the advertised price throughout a reasonable spread of the travel period, the ad was misleading.
The omission of "fares from" was also ruled to be misleading. The third issue about the failure to include the credit or debit card handling fee was not upheld because the ASA considered that it was not a compulsory charge that needed to be included in the headline price. Customers could choose another payment method, which did not incur a charge.