The agency explained that it had offered the fares as part of a "Flash Sale" , funded by STA Travel, on March 9 and that it had made it clear in an email to customers that there were only a limited number of seats available at that price.

In fact, they sold out within two hours, after which the price reverted back to £599.

It said its website had been updated to show the fare increase to £599 along with a message that stated "flash sale sold out" as soon as the £399 seats were gone. It also posted regular updates regarding availability on Facebook and Twitter.

However, the Advertising Standards Authority upheld the customers' compliant, saying that STA Travel had exaggerated the likelihood of consumers being able to obtain the advertised fare.

"We noted that the "Flash Sale" had been funded by STA Travel and that the availability of the £399 fares was therefore at their discretion," it said. "We considered that the number of fares available at £399 had been very limited and we therefore considered that this constituted material information that consumers needed to understand about the limitations of the offer.

"Although we acknowledged that the advertised fare was valid on the day and time specified in the e-mail and although we appreciated that STA Travel had attempted to make consumers aware of the limitations on availability, because information regarding the specific number of fares available at £399 had been omitted, we considered that the e-mail exaggerated the likelihood of consumers being able to obtain the advertised fare and concluded that it was likely to mislead."

The ASA ordered STA Travel not to exaggerate the likelihood of consumers being able to take advantage of offers in future campaigns