The retailer made its claim on July 17, coinciding with official figures published by waste watchdog the Waste and Resources Action Programme (Wrap) about the seven leading supermarkets' reduction of plastic bags.
Wrap's figures stated that the supermarkets had just failed to meet a target for cutting monthly plastic bag distribution by 50% in the three years to May 2009.
Participating retailers (Tesco, Asda, Sainsbury's, Marks & Spencer, Waitrose, Somerfield and the Co-operative Group) reduced the number of plastic bags by 48%, from 870m in May 2006 to 420m in May 2009.
Tesco's own statement claimed: "Our customers are now using more than 50% fewer carrier bags than they did before."
However, Tesco admitted to The Times that it had adjusted its figure to account for the growth in its sales and it did not reflect the actual reduction in the number of bags. The supermarket told Brand Republic it is standing by the methodology.
The Times also quotes a senior source at Tesco as saying the actual reduction had been significantly less than 50% but the company was concerned about negative publicity.
A Tesco spokesman told Brand Republic: "In working out how many carrier bags our customers are saving, it is misleading simply to compare the overall number of bags issued now with those issued in 2006.
"More customers now shop with Tesco than did in 2006, and on average each is using 50% fewer bags than they did then. This is the most meaningful number. We believe this is a huge achievement in three years."
Tesco's plastic bag reduction strategy involves encouraging consumers to cut the number of bags they use, by tactics such as giving them Clubcard points for bag re-use, rather than charging them for bags.