Tesco has been forced to put a cap on its Double the Difference price pledge after super-savvy shoppers exploited the deal to actually make money while doing the weekly shop.
Consumers were guided by a new and anonymous website, pricecheck.logicalinternetsolutions.com, that alerted them to items sold by Tesco at up to 66% more than Asda - which had enabled surfers to redeem vouchers in excess of £100 a time.
Tesco has now imposed a cap of £20 on any refunds. The retailer said the cap had been introduced to stop exploitation of the price pledge by "a very small number of consumers" and would enable it to keep its campaign running, benefiting the vast majority of its customers.
It is a mystery who is behind the website, which is registered to an address in Stockton-on-Tees. Calls made by The Grocer this week to numbers on registration documents went unanswered.
The website ranks the biggest savings by price and percentage when comparing Tesco and Asda side-by-side. It does not offer any information on how to manipulate Asda's 10% price guarantee, but solely targets Tesco.
This week it highlighted several savings which showed how customers could actually turn a substantial profit. With a saving of £5.71 on the Hardy's Crest Cabernet Shiraz Merlot, shoppers who bought nine bottles received a refund voucher worth £102.78.
Other killer deals ripe for exploitation included racks of Tesco Finest lamb, bottles of Baileys and even roasting tins.
"Doesn't wine taste so much better when it's free?" said a contributor to moneysavingexpert.com in a thread directing shoppers to the site.
A Tesco spokeswoman confirmed this week that the offer was being misused. "We were made aware via web forums that PriceCheck might not be being used for its genuine purpose by a small number of people," she said.
"Having looked into it, we've taken some steps to protect the initiative. We have now placed a cap of £20 on any refund entitlements: the vast majority of refunds have been nowhere near this figure. In fact, a very small number of customers receive any refund at all after they check their real shop against Asda because they see that Tesco is cheaper.
"Separately, we have had technical issues with a comparison feature of PriceCheck, which we continue to work on."
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