The Advertising Standards Authority (ASA) has found Ryanair guilty of misleading the public.
The ASA this morning upheld a complaint from easyJet who challenged the availability of £10 fares advertised for flights to Gothenburg and Dusseldorf-Weeze.
In its defense the Irish carrier said the offer was widely available during the booking period on the services from London Stansted.
It told the ASA fares had dropped from £10 to £5 the day before the offer started, and that thousands of seats were sold at the lower price.
ABTN understands Ryanair suggested this had an impact on the number of £10 fares available when the publicised offer started.
After evaluating the dispute the ASA confirmed Ryanair breached elements of the advertising standard CAP code, including substantiation, truthfulness, prices, availability of products and sales promotion rules.
An ASA statement said: "We noted that the ads stated £10 ONE WAY and considered that readers were likely to understand that this was a fixed price and that all seats were available at the advertised price.
"We understood that Ryanair had changed the offer to the fare of £5 one way just before the ad was published and noted they had provided [as evidence] one booking to Gothenburg and one to Dusseldorf Weeze that showed the base fare for each flight was £5.
"Although we considered those bookings indicated some flights may have been available at a lower price than the advertised £10 fare, we did not consider the evidence provided was sufficient to support a claim that all flights were available at £10 one way or at a lower, fixed, base fare."
"Consumers would understand £10 ONE WAY to mean that all flights to Gothenburg and Dusseldorf-Weeze were available at £10, and because we had not seen evidence to support that claim, we concluded that the ads were likely to mislead."
Paul Simmons, easyJet's UK general manager said: "We welcome the ASA's decision that one Ryanair advert broke six elements of the advertising standards code - that's something of a record, even for them."
The ASA said the adverts must not appear again in their current form and ordered the Irish company to ensure all seats were available at the quoted fare.
It said if seats were not available at the quoted fare the adverts should state prominently that were "from" a certain price.