Up to 40% of Tesco customers have felt let down by poor availability on promotions, a study has found.

The report by the Institute of Promotional Marketing comes as supermarkets find themselves under increasing scrutiny over their promotional tactics as the OFT prepares to unveil the results of its investigation into the way they advertise deals.

Asda was the second-worst at meeting customer demand, with 25% of shoppers who made a special trip to buy something on promotion unable to find it. Lidl and Aldi were next with 21% and 19% respectively. Of the big chains, Waitrose disappointed the least. Only 2% of its shoppers said a promotion they wanted was unavailable.

IPM insight director Colin Harper said retailers who didn't carry enough promotional stock were risking reputation and bottom line. "Out-of-stocks on promotions destroy faith in stores as well as brands," he said.

Verdict analyst Malcolm Pinkerton added: "If customers consistently can't get what they want they'll go elsewhere."

Tesco disputed the survey. "We measure availability, ­including promotions, against the shopping experience of millions of Tesco customers. Customer satisfaction with our availability is at its highest-ever level," said a spokesman.

According to consumer protection legislation, it is unfair to advertise a deal if the store does not believe it will be able to fulfil the anticipated demand. The OFT's final report into pricing is due out in December and will reveal how it intends to enforce these guidelines.

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