Tesco has moved swiftly to defend its latest price pledge after shoppers threatened on online forums to report the retailer to the Advertising Standards Authority.

Tesco kicked off a counter-offensive to Asda's 10% Price Guarantee last week, promising shoppers it would refund double the difference if their shopping would have been cheaper at Asda. However, The Grocer revealed that more than a third of prices highlighted in Tesco's adverts went up in price shortly before coming down. Shoppers used consumer website MoneySavingExpert.com to question the ads and complained about the limited range of lines ­covered.

"Has anyone nobbled Tesco yet about their extremely misleading advertising regarding this scheme? Or even contacted Advertising Standards or Trade Descriptions?" asked one post, while another said, "I think that it is false advertising if less than half the shop is compared".

Another added: "I spent £120 in Tesco on Monday. Only £45.88 of my Tesco shop was compared to Asda." Users also moaned about the voucher redemption process, calling the 36-hour wait for a result "tedious".

However, Tesco defended the pledge. "Approx-imately 60% of Tesco baskets were comparable in the first week," a spokesman said. "This is substantially better than Asda's mysupermarket.com basket. We have already conducted research and 84% of customers found it extremely/very/quite easy to use and 78% rated their ­experience positively."

The ASA said it had ­received no complaints.