The television campaign in March, created by The Red Brick Road, featured a voiceover that said: "You can now check the price of your shop at Tesco Price Check and if you find your comparable grocery shopping cheaper at Asda, we'll happily refund double the difference."
Clearcast said there was no maximum value for an individual Price Check voucher when it cleared the ad and if it knew Tesco was applying a £20 cap it would have advised the supermarket to include a disclaimer.
Tesco denied it misled customers after introducing the £20 limit on 24 March and defended itself by claiming the overwhelming majority taking part in the Price Check scheme were not affected by the £20 limit.
Some 15 people complained to the advertising watchdog about the £20 limit while Tesco provided data showing the highest number of transactions the price cap affected on any single day was over 1% of the total Price Check requests.
The ad watchdog noted the data initially submitted by Tesco showed the period between 20 June and 18 July, which came after Tesco changed the nature of the promotion.
On 29 April Tesco changed the conditions of the promotion so it would only pay the difference between the price of shopping at Tesco and Asda rather than doubling the difference.
The ASA requested extra information from Tesco for the entire promotional period, which showed the percentage of total Price Check requests that were affected by the £20 limit, from the beginning of the promotion to 29 April, was over 8.5%.
An additional claim by Tesco that tactical shoppers had threatened the viability of the offer was thrown out by the ad watchdog because there was no supporting evidence.
Consequently the ad campaign has been banned for not drawing attention to the £20 limit, which was likely to affect consumers' decisions to shop at Tesco during the promotion.
Tesco has been told to make sure that particularly significant conditions were included in the body of its advertising.
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