The Advertising Standards Agency (ASA) has criticised easyJet for making misleading claims about a flights sale last year.
The low-cost carrier claimed every seat would be discounted for two days in March 2007, with up to 25 per cent off prices, in the national press and on the internet.
However, two customers complained that on some of the flights, no discounts appeared to be available during the sale.
On investigation, easyJet claimed that as its fares increase as seats are sold, some of the flights included in the promotion would actually be more expensive than before the discounting period began as demand increased.
However, the rates would still be cheaper than normal, easyJet insisted.
The ASA disagreed, upholding the complaint.
In a statement, the watchdog said: "We considered that, because the ad stated 'Up to 25 per cent off every seat, every route, every day', most customers would expect fares to be cheaper than if they had booked tickets in the period immediately before the promotion began on 'every seat, every route, every day', for the whole of the promotional period.
"Because that was not the case we concluded that the ad was misleading."
The ASA instructed easyJet not to use the claim again unless it was qualified to make clear that, although discounted, prices might rise during the promotional period.
Rival budget airline Ryanair is also in trouble with the advertising regulator after ignoring the ASA on several issues, the company was last week referred to the Office of Fair Trading.