One complainant felt the online ad was misleading as the travel dates for the promotion were not included in the ad.
Another complainant was confused over a claim of a 20 per cent reduction off all seats during the promotional period as he understood the offer should have referred to "up to" 20 per cent off.
The ASA also challenged the ads for not making clear that price could increase during the promotional period.
Easyjet said the online ad had numerous indications that the promotion would last for a restricted time period.
It believed consumers would understand that references to summer in the artwork and copy meant the sale applied to travel during the summer period.
The online promotion took customers directly through to a landing page that contained information about the promotion and its travel dates.
Easyjet said, in reference to a previous ASA adjudication, it understood it to be acceptable for advertisers to provide significant conditions to an offer no more than one click-through away from online banner ads.
Easyjet confirmed the offer was for up to 20 per cent off rather than the 20 per cent off stated in the ad, it explained the phrase up to had been omitted in error and had not been broadcast for more than three days before it was corrected.
The ASA noted Easyjet’s argument the offer travel period was clear from the creative treatment of the promotion, however it considered consumers would understand "summer" to indicate any time including during the August summer holidays.
It concluded that the online ad was misleading and the travel dates should have been included where the online promotion was introduced to the consumer on the advertiser’s homepage.
The previous adjudication to which Easyjet had referred related to a banner ad in paid-for-space on a third-party website, it said.
Although the radio ad had been replaced with the correct information, consumers who heard the incorrect ad could have been misinformed about the offer and for that reason the regulator deemed it misleading.
The ASA ruled that the ads must not appear again in their current forms.