It is the latest proof to come courtesy of budget airline advertising departments that when a deal seems too good to be true, it usually is.

The no-frills carrier easyJet has been slapped on the wrist for a misleading promotion last summer that promised to refund customers double the difference if they could find a cheaper deal elsewhere.

The Advertising Standards Authority (ASA) ordered the airline to scrap the campaign after it found that small-print restrictions tighter than a £19.99 window seat made it practically impossible for passengers to take the airline up on its offer. The advert also prompted cries of foul play from Ryanair, a regular offender of advertising guidelines and a constant critic of the ASA.

Declaring that “our best summer fares are disappearing fast”, Easyjet’s internet adverts challenged customers to find flights cheaper than the £19.99 single fares over a five-day period in June 2007.

The advert, which featured a sandcastle sinking through an orange egg-timer, promised to pay out double the difference to those who did.

To get the refund, however, not only would rival flights have to share exactly the same departure and destination airports, they would also have to depart within an hour of each other.

There was one more catch — all claims had to be made within one hour of booking the easyJet flights, meaning customers had to find cheaper deals quicker than you can say Alicante. Even if they could find a flight that satisfied the stiff criteria, small print showed that the refund would be no more than £75.

The airline was unable to prove that a single customer had succeeded in meeting the restrictions, which left the ASA to conclude that it was an almost impossible task.

A spokesman said: “We considered that while the design and layout of the ads gave the impression the double-the-difference refund was easy to obtain, the very restricting terms and conditions suggested it was not.

“We also noted easyJet had not sent evidence to show customers had been able to take up the offer. We therefore concluded the ads were misleading.”

The ASA found that the no-frills carrier contravened four separate clauses of the code; 3.1 (Substantiation), 7.1 (Truthfulness), 17.1 (Guarantees) and 19.1 (Other comparisons).

It is the third time that the ASA has upheld a complaint against easyJet in the past year. It is also a rare victory for Ryanair, the rival airline that brought the complaint, and which has been reprimanded by the same watchdog twice as many times over the same period.

Peter Sherrard, a spokesman for Ryanair, denied the use of the ASA was hypocritical. “We couldn’t care less about any ruling, good, bad or indifferent, from the absolutely stupid asses at the ASA,” he told The Times.

A spokeswoman for easyJet said: “We are obviously disappointed by the decision as we felt the offer was reasonable. The easyJet advert was judged accurate and it was only the accessibility of the price promise the ASA disagreed with.”

Times Recommends

• Lies and cover-up at MG Rover exposed by report
• High oil prices push manufacturing costs up
• Primark founder steps down as chief executive