Sainsbury's has been ordered to change its "brand match" campaign after the advertising watchdog found it was misleading consumers.
The television, radio, internet and newspaper campaign promised customers would not pay more for brands than at Asda or Tesco, with the supermarket giant giving a voucher if prices were found to be cheaper at either of its two rivals.
The Advertising Standards Authority (ASA) received 20 complaints, from Tesco and 19 members of the public, that the coupon would in some cases confirm that customers would have paid less for branded goods at the two other chains.
Some complained that the ads did not make it clear that any saving was calculated across all of the brands in a shopper's basket or that the offer was only available to those who spent £20 or more.
Sainsbury's said it strongly believed that the brand match was a "genuine, clear and concise offer" that relied on a simple process.
The supermarket said the offer was straightforward for consumers and had been well received, with nearly 100 million coupons issued, indicating that shoppers understood the deal from the advertising.
The ASA noted that consumers would need to make a further purchase in order to redeem the coupons, which were limited to a value of £10 and had to be redeemed within two weeks.
It also understood that prices were only matched rather than bettered, and that consumers would in some instances pay more in order to receive the coupon for the difference.
The ASA ruled that the ads must not appear again in their current form and added: "We told Sainsbury's to ensure future ads did not imply consumers would not pay more, or would save money, if that was not the case.
"We also told them to ensure all significant conditions of promotions were made clear in future."
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Sainsbury's said: "Sainsbury's is committed to providing advertising that our customers can easily understand. We do not believe that our customers have been misled but we have already changed our current advertising to reflect the concerns raised."
