Ryanair has been ordered by Britain's Advertising Standards Authority not to repeat a TV commercial offering seats for free.
The budget airline's fare didn't include non-optional extra taxes and charges, the Advertising Standards ASA said.
Ryanair's "amazing autumn sale" commercials promised "Four Million Zero Fare Seats". Small print at the bottom of the screen said taxes and charges applied. Those costs ranged from £11.70 (€17.50) to £21.70 (€32.40) across 20 UK airports.
A voice-over said: "Ryanair, Britain's biggest passenger airline, has released four million zero fare seats on its website in an amazing autumn sale." It added: "All fares under this offer are zero. Only taxes and charges are payable and we guarantee no fuel surcharges."
Monarch Airlines complained to the ASA about the commercial. After an investigation, the industry watchdog found Ryanair in breach of advertising rules relating to pricing requirements.
It said the lowest amount payable under the terms of the promotion was £11.70. For that reason the airline could have said seats cost "from" that amount - and made clear that the charge varied according to the different airports.
"Because the non-optional charges were not impossible to quantify, we considered that the ad was in breach of the Code," the industry watchdog said.
Ryanair had no immediate comment.
The airline has fallen foul of the ASA before.
In May, the watchdog upheld a complaint about an advert for three million free tickets because it failed to make clear that the offer did not include flights on Fridays.
In 2004, it rapped a promotion offering "giveaway" flights because passengers would have to pay charges and taxes.
Agencies