Easyjet had defended its advert

The Advertising Standards Agency said the ads broke rules on truthfulness, guarantees and substantiation, after a complaint was brought by rival Ryanair.

The adverts stated: "Book by Friday and if you find it cheaper elsewhere we'll refund double the difference."

But it only applied to a restricted selection and number of flights.

The deal was only available on flights leaving within an hour of Easyjet services, out of the same airport and to the same destination as their flights.

The ASA has said Easyjet cannot re-use the adverts, which appeared on the internet and in newspapers during summer 2007.

'Misleading'

The Luton-based budget airline said the terms and conditions were stated on each advertisement and were not misleading, and said many passengers had taken advantage of the offer.

However, the ASA ruled: "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 sufficient evidence to show customers had been able to take up the offer. We therefore concluded the ads were misleading."

Ryanair had also complained that other adverts implied that Easyjet's flights were always cheaper than those of their rivals.

But the ASA ruled these adverts did not imply Easyjet's fares were always the cheapest, as passengers had to find a lower price on a comparable flight to take advantage of this particular offer.